[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_dev.go

@@ -20,7 +20,7 @@ func ClientQUICTLSConfig() *tls.Config {
 
 	return &tls.Config{
 		InsecureSkipVerify: true,             // Debug mode allows insecure connections
-		NextProtos:         []string{nbalpn}, // Ensure this matches the server's ALPN
+		NextProtos:         []string{NBalpn}, // Ensure this matches the server's ALPN
 		RootCAs:            certPool,
 	}
 }