feat(route): add support for relaying PROXY protocol header to TCP upstreams

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

internal/route/stream/tcp_tcp.go

@@ -25,13 +25,15 @@ type TCPTCPStream struct {
 	dst   *net.TCPAddr
 	agent *agentpool.Agent
 
+	relayProxyProtocolHeader bool
+
 	preDial nettypes.HookFunc
 	onRead  nettypes.HookFunc
 
 	closed atomic.Bool
 }
 
-func NewTCPTCPStream(network, dstNetwork, listenAddr, dstAddr string, agent *agentpool.Agent) (nettypes.Stream, error) {
+func NewTCPTCPStream(network, dstNetwork, listenAddr, dstAddr string, agent *agentpool.Agent, relayProxyProtocolHeader bool) (nettypes.Stream, error) {
 	dst, err := net.ResolveTCPAddr(dstNetwork, dstAddr)
 	if err != nil {
 		return nil, err
@@ -40,7 +42,14 @@ func NewTCPTCPStream(network, dstNetwork, listenAddr, dstAddr string, agent *age
 	if err != nil {
 		return nil, err
 	}
-	return &TCPTCPStream{network: network, dstNetwork: dstNetwork, laddr: laddr, dst: dst, agent: agent}, nil
+	return &TCPTCPStream{
+		network:                  network,
+		dstNetwork:               dstNetwork,
+		laddr:                    laddr,
+		dst:                      dst,
+		agent:                    agent,
+		relayProxyProtocolHeader: relayProxyProtocolHeader,
+	}, nil
 }
 
 func (s *TCPTCPStream) ListenAndServe(ctx context.Context, preDial, onRead nettypes.HookFunc) error {
@@ -158,6 +167,14 @@ func (s *TCPTCPStream) handle(ctx context.Context, conn net.Conn) {
 	if s.closed.Load() {
 		return
 	}
+	if s.relayProxyProtocolHeader {
+		if err := writeProxyProtocolHeader(dstConn, conn); err != nil {
+			if !s.closed.Load() {
+				logErr(s, err, "failed to write proxy protocol header")
+			}
+			return
+		}
+	}
 
 	src := conn
 	dst := dstConn