Add `relay_proxy_protocol_header` configuration option for TCP routes that enables
forwarding the original client IP address to upstream services via PROXY protocol
v2 headers. This feature is only available for TCP routes and includes validation
to prevent misuse on UDP routes.
- Add RelayProxyProtocolHeader field to Route struct with JSON tag
- Implement writeProxyProtocolHeader in stream package to craft v2 headers
- Update TCPTCPStream to conditionally send PROXY header to upstream
- Add validation ensuring feature is TCP-only
- Include tests for both enabled/disabled states and incoming proxy header relay
