[core] Improve DX by providing a Codable API for Streaming Lambda

[sebsto]

Currently, users of LambdaStreamingHandler only have one protocol available that accepts a event: ByteBuffer

swift-aws-lambda-runtime/Sources/AWSLambdaRuntime/LambdaHandlers.swift

Line 42 in bf2385a

responseWriter: some LambdaResponseStreamWriter,

Contrary to regular handlers, it's up to the developer to convert the ByteBuffer to a Codable struct in the very likely case the function accepts a JSON event.

I propose to add Codable support to Streaming lambda, just like we did for non streaming Lambda handlers.

[sebsto]

This is implemented by #532