[self-hosted] add netbird server (#5232)

* Unified NetBird combined server (Management, Signal, Relay, STUN) as a single executable with richer YAML configuration, validation, and defaults.
  * Official Dockerfile/image for single-container deployment.
  * Optional in-process profiling endpoint for diagnostics.
  * Multiplexing to route HTTP/gRPC/WebSocket traffic via one port; runtime hooks to inject custom handlers.
* **Chores**
  * Updated deployment scripts, compose files, and reverse-proxy templates to target the combined server; added example configs and getting-started updates.

relay/server/server.go

@@ -3,6 +3,7 @@ package server
 import (
 	"context"
 	"crypto/tls"
+	"net"
 	"net/url"
 	"sync"
 
@@ -134,3 +135,10 @@ func (r *Server) ListenerProtocols() []protocol.Protocol {
 func (r *Server) InstanceURL() url.URL {
 	return r.relay.InstanceURL()
 }
+
+// RelayAccept returns the relay's Accept function for handling incoming connections.
+// This allows external HTTP handlers to route connections to the relay without
+// starting the relay's own listeners.
+func (r *Server) RelayAccept() func(conn net.Conn) {
+	return r.relay.Accept
+}