feat: complete header forwarding implementation with pass-through by default

- Implement pass-through by default behavior for header forwarding
- Add optional filtering via agent manifest custom_headers configuration
- Update FastACP server to handle new request.headers structure properly
- Rewrite test suite to match new pass-through behavior and API structure
- Update all documentation to reflect new architecture and API
- Remove size/count limits and simplify configuration schema

Key changes:
- All headers forwarded unless agent has specific filtering config
- Clean request.headers API structure for extensibility
- Wildcard pattern support for flexible header matching
- Zero configuration required for basic header forwarding

Breaking changes: Replaced extra_headers with request.headers structure

Author: prassanna-ravishankar