[server] Add health check HTTP endpoint for Relay server (#4297)

The health check endpoint listens on a dedicated HTTP server.
By default, it is available at 0.0.0.0:9000/health. This can be configured using the --health-listen-address flag.

The results are cached for 3 seconds to avoid excessive calls.

The health check performs the following:

Checks the number of active listeners.
Validates each listener via WebSocket and QUIC dials, including TLS certificate verification.

shared/relay/tls/client_prod.go

@@ -19,7 +19,7 @@ func ClientQUICTLSConfig() *tls.Config {
 	}
 
 	return &tls.Config{
-		NextProtos: []string{nbalpn},
+		NextProtos: []string{NBalpn},
 		RootCAs:    certPool,
 	}
 }