Support Lambda response streaming

It would be great if LLRT supported Lambda response streaming as per https://docs.aws.amazon.com/lambda/latest/dg/runtimes-custom.html#runtimes-custom-response-streaming.

Start with the linked AWS Lambda custom runtime response-streaming documentation to understand the required behavior. Then locate LLRT's Lambda runtime integration and determine how streaming responses should be represented; done means LLRT supports the documented response-streaming flow and verifies it with an appropriate runtime test.

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