Add proxy response type

[Athius]

Summary

This PR updates the gateway to the newer Lambda and HTTP client/server dependency stack, adds
configurable upstream response decoding, and refreshes the project documentation.

Changes

• update aws_lambda_events, base64, hyper, and reqwest
• migrate the gateway implementation to hyper 1.x APIs
• add --response-type / RESPONSE_TYPE with support for:
  • auto (default)
  • proxy
  • v2
• auto-detect Lambda response format by inspecting the upstream JSON payload
• keep explicit proxy and v2 modes for callers that want to force a format
• refresh the Dockerfile base image and environment variable syntax
• update README.md with the new runtime options and Docker examples
• add AGENTS.md contributor guidelines for this repository

Notes

• auto mode currently treats responses with a top-level cookies field as
  ApiGatewayV2httpResponse
• responses without cookies are decoded as ApiGatewayProxyResponse

Testing

• cargo fmt --all -- --check
• cargo check
• cargo test
• cargo clippy -- -D warnings