add: HTTP trailers support implementation plan

This document outlines the comprehensive plan for implementing HTTP trailers
support in @fastify/reply-from, including:

- Architecture design for trailer collection across HTTP/1.1, HTTP/2, and undici clients
- Plugin integration with new configuration options and hooks
- Security considerations with forbidden trailer filtering
- Comprehensive testing strategy and timeline
- Backward-compatible implementation approach

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: mcollina