Implement x402-extensions crate with generic Extension<T>, bazaar, and sign-in-with-x extensions

[Copilot]

Per the x402 v2 spec, extensions are a key-value map in PaymentRequired/PaymentPayload where each value has info, schema, and optional extra fields. This PR introduces a typed extension system and implements the bazaar (resource discovery) and sign-in-with-x extensions.

x402-core — Extension type refactoring

• Extension becomes Extension<T = AnyJson> — backward compatible, transport types unchanged
• Added extra: Record<AnyJson> with custom Serialize/Deserialize that flattens extra fields alongside info/schema
• ExtensionInfo trait: const ID for map key + fn schema() for compile-time JSON Schema generation
• ExtensionMapInsert trait: ergonomic insert_typed() on Record<Extension>
• Conversion methods: Extension::typed(), into_pair(), into_typed(), with_extra()

x402-extensions — New crate

• bazaar: BazaarInfo with discriminated union BazaarInput (Http | Mcp tagged on type), BazaarHttpInput (all HTTP methods), BazaarMcpInput (tool name, inputSchema, transport), BazaarOutput. Schema auto-generated via schemars 1.0.
• sign_in_with_x: SignInWithXInfo (domain, uri, nonce, timestamps, etc.) + SupportedChain helper for the supportedChains extra field.

x402-kit — Re-export

• pub mod extensions re-exports x402-extensions.

Usage

use x402_extensions::bazaar::*;
use x402_core::types::{Extension, ExtensionMapInsert, Record};

// Typed construction with auto-generated schema
let ext = Extension::typed(BazaarInfo::builder()
    .input(BazaarInput::Mcp(BazaarMcpInput::builder()
        .tool("financial_analysis")
        .input_schema(json!({"type": "object", "properties": {"ticker": {"type": "string"}}}))
        .build()))
    .build());

// Insert into transport map
let mut extensions: Record<Extension> = Record::new();
extensions.insert_typed(ext);

// Extra fields (e.g. supportedChains) flatten during serialization
let ext = Extension::typed(sign_in_info)
    .with_extra("supportedChains", json!([{"chainId": "eip155:8453", "type": "eip191"}]));

Users can implement their own extensions without upstream changes by implementing ExtensionInfo on any Clone + 'static type.

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