[core] Implement Lambda streaming with custom HTTP headers

[sebsto]

Fix #520

[Copilot]

Pull Request Overview

This PR implements Lambda streaming functionality with custom HTTP headers, allowing developers to set HTTP status codes and headers before streaming response bodies. This addresses the need for more control over HTTP response metadata in streaming Lambda functions.

• Adds StreamingLambdaStatusAndHeadersResponse struct for configuring HTTP status and headers
• Implements writeStatusAndHeaders extension method on LambdaResponseStreamWriter
• Updates documentation and examples to demonstrate the new streaming capabilities

Reviewed Changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
readme.md	Adds documentation and examples for streaming with HTTP headers
Sources/AWSLambdaRuntime/LambdaResponseStreamWriter+Headers.swift	Core implementation of headers functionality with response struct and extension methods
Tests/AWSLambdaRuntimeTests/LambdaResponseStreamWriter+HeadersTests.swift	Comprehensive test suite covering functionality, error handling, and integration scenarios
Examples/Streaming/Sources/main.swift	Updated streaming examples demonstrating headers usage and conditional responses
Examples/Streaming/README.md	Enhanced documentation explaining header usage and streaming patterns