[WIP] DO NOT MERGE - [core] Remove @unchecked Sendable from Lambda Runtime (depends on PR#508)

[sebsto]

This PR depends on #508

When removing the NIOLockedBox, we wanted to remove the last @unchecked Sendable in the code.

Motivation:

There is no good reason to bypass compiler verifications with @unchecked Sendable in this scenario. We would like to bring the extra verifications' benefit to the user of the library.

Modifications:

Lambda handlers and all depending structs and functions are tagged @sendable

Result:

This adds a requirement on user defined handlers (the public API of this library).
Code like

let handler:
    (Event, LambdaContext) async throws -> Response = {
        (request: Event, context: LambdaContext) in
    ...
    return Response() 
}
let runtime = LambdaRuntime(body: handler) 
try await runtime().run() 

is no longer accepted. The handler must marked @Sendable

let handler:
    @Sendable (Event, LambdaContext) async throws -> Response = {
        (request: Event, context: LambdaContext) in
    ...
    return Response() 
}

Fortunately, thanks to the compiler type inference, this does not affect handlers passed as a closure, such as

let runtime = LambdaRuntime {
    (event: HelloRequest, context: LambdaContext) in

    HelloResponse(
        greetings: "Hello \(event.name). You look \(event.age > 30 ? "younger" : "older") than your age."
    )
}

As we expect this is the preferred way to write a handler, we think the change in the public API is acceptable. Only more advanced users will have to explicitly add the @Sendable conformance to their standalone handler functions.

[sebsto]

We decided to not force Sendable on Handler. This commit should allow that
7b50167