[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/server_prod.go

@@ -12,6 +12,6 @@ func ServerQUICTLSConfig(originTLSCfg *tls.Config) (*tls.Config, error) {
 		return nil, fmt.Errorf("valid TLS config is required for QUIC listener")
 	}
 	cfg := originTLSCfg.Clone()
-	cfg.NextProtos = []string{nbalpn}
+	cfg.NextProtos = []string{NBalpn}
 	return cfg, nil
 }