Add new handler protocols + Codable support

[aryan-25]

Motivation:

As part of the implementation of the v2 API (#339), we want to add the handler protocols LambdaHandler and LambdaWithBackgroundProcessingHandler.

Modifications:

• Added the two remaining handler protocols: LambdaHandler and LambdaWithBackgroundProcessingHandler.

  • Note: LambdaHandler is currently temporarily named as NewLambdaHandler as there is already an existing LambdaHandler. This will be sorted out closer to release.

• Added LambdaHandlerAdapter and LambdaCodableAdapter to allow the two new handler protocols to work with the LambdaRuntime (as LambdaRuntime requires a StreamingLambdaHandler conforming handler).

  • LambdaHandlerAdapter:
    • Adapts a LambdaHandler conforming handler to conform to LambdaWithBackgroundProcessingHandler by passing the output returned from LambdaHandler to LambdaWithBackgroundProcessingHandler's output writer.
  • LambdaCodableAdapter:
    • Adapts a LambdaWithBackgroundProcessingHandler conforming handler to conform to StreamingLambdaHandler.
    • First decodes the received ByteBuffer event into LambdaWithBackgroundProcessingHandlers generic Event object, and passes this to its handle(...) function.
    • Encodes the output written to LambdaResponseWriter into a ByteBuffer and send this to StreamingLambdaHandler's LambdaResponseStreamWriter.

• Added StreamingClosureHandler and ClosureHandler structs conforming to StreamingLambdaHandler and LambdaHandler protocols respectively.

  • This allows a Lambda handler to be created by simply writing the handler in the form of a closure, as compared to first creating an object conforming to the handler protocol and then writing the handle(...) function.

Result:

• All three handler protocols are introduced.
• Handlers conforming to the LambdaHandler and LambdaWithBackgroundProcessingHandler can be used with the new LambdaRuntime through the adapters.
• Handlers can be defined in a clean manner through the closure handlers.

[fabianfett]

Great progress! We need docs, docs and docs. Take them from the proposal!

[fabianfett]

API breakage check also reports on package access modifiers:

2 breaking changes detected in AWSLambdaRuntimeCore:
  💔 API breakage: protocol LambdaRuntimeClientProtocol has generic signature change from <Self.Writer : AWSLambdaRuntimeCore.LambdaResponseStreamWriter> to <Self.Writer : AWSLambdaRuntimeCore.LambdaRuntimeClientResponseStreamWriter>
  💔 API breakage: func LambdaResponseStreamWriter.reportError(_:) has been removed

[fabianfett]

Looks great! Thank you!