[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

client/ui/client_ui.go

@@ -280,7 +280,7 @@ func newServiceClient(addr string, logFile string, a fyne.App, showSettings bool
 
 		showAdvancedSettings: showSettings,
 		showNetworks:         showNetworks,
-		update:               version.NewUpdate(),
+		update:               version.NewUpdate("nb/client-ui"),
 	}
 
 	s.eventHandler = newEventHandler(s)