feat: restructure header forwarding API from extra_headers to request.headers

- Update CustomHeadersConfig schema to remove max_header_size and max_headers_count
- Change API from flat extra_headers parameter to nested request.headers structure
- Implement pass-through by default behavior (headers forwarded unless filtered)
- Update all ACP service methods, temporal activities, and ADK modules
- Remove size/count limits from header filtering logic

Breaking changes: extra_headers parameter replaced with request parameter across all ACP APIs

Author: prassanna-ravishankar