Fix Sendable checking

[stevapple]

Current state:

Sources/AWSLambdaRuntimeCore/LambdaContext.swift:28:20: error: stored property 'logger' of 'Sendable'-conforming struct 'InitializationContext' has non-sendable type 'Logger'
        public let logger: Logger
                   ^
Sources/AWSLambdaRuntimeCore/LambdaContext.swift:34:20: error: stored property 'eventLoop' of 'Sendable'-conforming struct 'InitializationContext' has non-sendable type 'EventLoop'
        public let eventLoop: EventLoop
                   ^
Sources/AWSLambdaRuntimeCore/LambdaContext.swift:37:20: error: stored property 'allocator' of 'Sendable'-conforming struct 'InitializationContext' has non-sendable type 'ByteBufferAllocator'
        public let allocator: ByteBufferAllocator
                   ^
Sources/AWSLambdaRuntimeCore/LambdaContext.swift:98:17: error: stored property 'storage' of 'Sendable'-conforming struct 'LambdaContext' has non-sendable type 'LambdaContext._Storage'
    private var storage: _Storage
                ^

We’re waiting for SwiftNIO and SwiftLog to fully support Sendable, meanwhile the design of LambdaContext should be adapted for Sendable.

---

WIP: Fix Sendable checking to pass nightly CI.

Motivation:

Add necessary Sendable conformance according to Swift Concurrency adoption guidelines for Swift Server Libraries (swift-server/guides#70).

Modifications:

• Introduce LambdaSendable as a drop-in Sendable replacement that's compatible with prior Swift versions;
• Conform LambdaHandler to Sendable;
• Conform LambdaContext and Lambda.InitializationContext to LambdaSendable;
• Require EventLoopLambdaHandler.Event and EventLoopLambdaHandler.Output to conform to LambdaSendable.

[swift-server-bot]

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[stevapple]

I have only added necessary Sandable conformances for passing the compiler’s check. If there’s other type/protocol that should also conform to Sendable please let me know.

[fabianfett]

@swift-server-bot test this please