[management] check and log on new management version (#4029)

This PR enhances the version checker to send a custom User-Agent header when polling for updates, and configures both the management CLI and client UI to use distinct agents. 

- NewUpdate now takes an `httpAgent` string to set the User-Agent header.
- `fetchVersion` builds a custom HTTP request (instead of `http.Get`) and sets the User-Agent.
- Management CLI and client UI now pass `"nb/management"` and `"nb/client-ui"` respectively to NewUpdate.
- Tests updated to supply an `httpAgent` constant.
- Logs if there is a new version available for management

management/cmd/management.go

@@ -357,6 +357,13 @@ var (
 			log.WithContext(ctx).Infof("running HTTP server and gRPC server on the same port: %s", listener.Addr().String())
 			serveGRPCWithHTTP(ctx, listener, rootHandler, tlsEnabled)
 
+			update := version.NewUpdate("nb/management")
+			update.SetDaemonVersion(version.NetbirdVersion())
+			update.SetOnUpdateListener(func() {
+				log.WithContext(ctx).Infof("your management version, \"%s\", is outdated, a new management version is available. Learn more here: https://github.com/netbirdio/netbird/releases", version.NetbirdVersion())
+			})
+			defer update.StopWatch()
+
 			SetupCloseHandler()
 
 			<-stopCh