Refactor the Swift Settings in Package.swift (#558)

- Use the new Swift 6 `@available` macro to remove requirement on
`.platform` in Package.swift.
- DRY: define the swift settings once for all in `Package.swift`

### Motivation:

- Remove the requirement to build on macOS 15 in `Package.swift`. This
allows library builders and end users to be more flexible on their
dependency requirements.
- The code is optionally compiled on macOS 15 and Linux, but SPM don't
enforce it anymore.
- Avoid repeating ourself. Be sure the same settings are applied on all
targets.

### Modifications:

- Create a `var swiftSetting: [SwiftSettings]` and reuse it for all
targets.
- Use `AvailabilityMacro=LambdaSwift 2.0:macOS 15.0`
- Add this on top of the majority struct / classes
```swift
#if swift(>=6.1)
@available(LambdaSwift 2.0, *)
#endif
```
### Result:

When using Swift 6.1, there is no more SPM dependency on macOS 15

Author: sebsto

Package.swift

@@ -2,9 +2,15 @@
 
 import PackageDescription
 
+let defaultSwiftSettings: [SwiftSetting] =
+    [
+        .enableExperimentalFeature(
+            "AvailabilityMacro=LambdaSwift 2.0:macOS 15.0"
+        )
+    ]
+
 let package = Package(
     name: "swift-aws-lambda-runtime",
-    platforms: [.macOS(.v15)],
     products: [
         .library(name: "AWSLambdaRuntime", targets: ["AWSLambdaRuntime"]),
         // plugin to package the lambda, creating an archive that can be uploaded to AWS
@@ -43,7 +49,8 @@ let package = Package(
                     package: "swift-service-lifecycle",
                     condition: .when(traits: ["ServiceLifecycleSupport"])
                 ),
-            ]
+            ],
+            swiftSettings: defaultSwiftSettings
         ),
         .plugin(
             name: "AWSLambdaPackager",
@@ -67,8 +74,10 @@ let package = Package(
                 .byName(name: "AWSLambdaRuntime"),
                 .product(name: "NIOTestUtils", package: "swift-nio"),
                 .product(name: "NIOFoundationCompat", package: "swift-nio"),
-            ]
+            ],
+            swiftSettings: defaultSwiftSettings
         ),
+
         // for perf testing
         .executableTarget(
             name: "MockServer",
@@ -77,7 +86,8 @@ let package = Package(
                 .product(name: "NIOHTTP1", package: "swift-nio"),
                 .product(name: "NIOCore", package: "swift-nio"),
                 .product(name: "NIOPosix", package: "swift-nio"),
-            ]
+            ],
+            swiftSettings: defaultSwiftSettings
         ),
     ]
 )

Package@swift-6.0.swift

@@ -2,9 +2,17 @@
 
 import PackageDescription
 
+let defaultSwiftSettings: [SwiftSetting] = [
+    .define("FoundationJSONSupport"),
+    .define("ServiceLifecycleSupport"),
+    .define("LocalServerSupport"),
+    .enableExperimentalFeature(
+        "AvailabilityMacro=LambdaSwift 2.0:macOS 15.0"
+    ),
+]
+
 let package = Package(
     name: "swift-aws-lambda-runtime",
-    platforms: [.macOS(.v15)],
     products: [
         .library(name: "AWSLambdaRuntime", targets: ["AWSLambdaRuntime"]),
         // plugin to package the lambda, creating an archive that can be uploaded to AWS
@@ -28,11 +36,7 @@ let package = Package(
                 .product(name: "NIOPosix", package: "swift-nio"),
                 .product(name: "ServiceLifecycle", package: "swift-service-lifecycle"),
             ],
-            swiftSettings: [
-                .define("FoundationJSONSupport"),
-                .define("ServiceLifecycleSupport"),
-                .define("LocalServerSupport"),
-            ]
+            swiftSettings: defaultSwiftSettings
         ),
         .plugin(
             name: "AWSLambdaPackager",
@@ -57,11 +61,7 @@ let package = Package(
                 .product(name: "NIOTestUtils", package: "swift-nio"),
                 .product(name: "NIOFoundationCompat", package: "swift-nio"),
             ],
-            swiftSettings: [
-                .define("FoundationJSONSupport"),
-                .define("ServiceLifecycleSupport"),
-                .define("LocalServerSupport"),
-            ]
+            swiftSettings: defaultSwiftSettings
         ),
         // for perf testing
         .executableTarget(
@@ -71,7 +71,8 @@ let package = Package(
                 .product(name: "NIOHTTP1", package: "swift-nio"),
                 .product(name: "NIOCore", package: "swift-nio"),
                 .product(name: "NIOPosix", package: "swift-nio"),
-            ]
+            ],
+            swiftSettings: defaultSwiftSettings
         ),
     ]
 )

Sources/AWSLambdaRuntime/ControlPlaneRequest.swift

@@ -15,20 +15,23 @@
 import NIOCore
 import NIOHTTP1
 
+@available(LambdaSwift 2.0, *)
 enum ControlPlaneRequest: Hashable {
     case next
     case invocationResponse(String, ByteBuffer?)
     case invocationError(String, ErrorResponse)
     case initializationError(ErrorResponse)
 }
 
+@available(LambdaSwift 2.0, *)
 enum ControlPlaneResponse: Hashable {
     case next(InvocationMetadata, ByteBuffer)
     case accepted
     case error(ErrorResponse)
 }
 
 @usableFromInline
+@available(LambdaSwift 2.0, *)
 package struct InvocationMetadata: Hashable, Sendable {
     @usableFromInline
     package let requestID: String

Sources/AWSLambdaRuntime/ControlPlaneRequestEncoder.swift

@@ -14,6 +14,7 @@
 
 import NIOCore
 
+@available(LambdaSwift 2.0, *)
 struct ControlPlaneRequestEncoder: _EmittingChannelHandler {
     typealias OutboundOut = ByteBuffer
 

Sources/AWSLambdaRuntime/FoundationSupport/Context+Foundation.swift

@@ -19,6 +19,7 @@ import FoundationEssentials
 import struct Foundation.Date
 #endif
 
+@available(LambdaSwift 2.0, *)
 extension LambdaContext {
     /// Returns the deadline as a Date for the Lambda function execution.
     /// I'm not sure how usefull it is to have this as a Date, with only seconds precision,

Sources/AWSLambdaRuntime/FoundationSupport/Lambda+JSON.swift

@@ -59,6 +59,7 @@ public struct LambdaJSONOutputEncoder<Output: Encodable>: LambdaOutputEncoder {
     }
 }
 
+@available(LambdaSwift 2.0, *)
 extension LambdaCodableAdapter {
     /// Initializes an instance given an encoder, decoder, and a handler with a non-`Void` output.
     ///   - Parameters:
@@ -84,6 +85,7 @@ extension LambdaCodableAdapter {
     }
 }
 
+@available(LambdaSwift 2.0, *)
 extension LambdaRuntime {
     /// Initialize an instance with a `LambdaHandler` defined in the form of a closure **with a non-`Void` return type**.
     /// - Parameters:

Sources/AWSLambdaRuntime/Lambda+Codable.swift

@@ -48,6 +48,7 @@ public struct VoidEncoder: LambdaOutputEncoder {
 }
 
 /// Adapts a ``LambdaHandler`` conforming handler to conform to ``LambdaWithBackgroundProcessingHandler``.
+@available(LambdaSwift 2.0, *)
 public struct LambdaHandlerAdapter<
     Event: Decodable,
     Output,
@@ -80,6 +81,7 @@ public struct LambdaHandlerAdapter<
 }
 
 /// Adapts a ``LambdaWithBackgroundProcessingHandler`` conforming handler to conform to ``StreamingLambdaHandler``.
+@available(LambdaSwift 2.0, *)
 public struct LambdaCodableAdapter<
     Handler: LambdaWithBackgroundProcessingHandler,
     Event: Decodable,

Sources/AWSLambdaRuntime/Lambda+LocalServer.swift

@@ -37,6 +37,7 @@ import Synchronization
 //         )
 //     }
 // }
+@available(LambdaSwift 2.0, *)
 extension Lambda {
     /// Execute code in the context of a mock Lambda server.
     ///
@@ -84,6 +85,7 @@ extension Lambda {
 /// 1. POST /invoke - the client posts the event to the lambda function
 ///
 /// This server passes the data received from /invoke POST request to the lambda function (GET /next) and then forwards the response back to the client.
+@available(LambdaSwift 2.0, *)
 internal struct LambdaHTTPServer {
     private let invocationEndpoint: String
 

Sources/AWSLambdaRuntime/Lambda.swift

@@ -29,6 +29,7 @@ import ucrt
 #error("Unsupported platform")
 #endif
 
+@available(LambdaSwift 2.0, *)
 public enum Lambda {
     @inlinable
     package static func runLoop<RuntimeClient: LambdaRuntimeClientProtocol, Handler>(
@@ -98,6 +99,7 @@ public enum Lambda {
 
 // MARK: - Public API
 
+@available(LambdaSwift 2.0, *)
 extension Lambda {
     /// Utility to access/read environment variables
     public static func env(_ name: String) -> String? {

Sources/AWSLambdaRuntime/LambdaClock.swift

@@ -51,6 +51,7 @@ import ucrt
 /// The Lambda execution environment uses UTC as a timezone,
 /// `LambdaClock` operates in UTC and does not account for time zones.
 /// see: TZ in https://docs.aws.amazon.com/lambda/latest/dg/configuration-envvars.html
+@available(LambdaSwift 2.0, *)
 public struct LambdaClock: Clock {
     public typealias Duration = Swift.Duration