[FR] Stacked/Grouped/Bundled notifications support for Android

### Feature proposal

* Firebase Component: Messaging

Hello! I didn't find a way to stack notifictions on Android using REST/HTTP API (like notifications from Gmail, for example). The only possible workaround is writing Java code that extends FirebaseMessagingService, but it's not an option for Unity devs who are not familiar with native Android programming. Can you make a new option in REST/HTTP API to add stacked notifications support please?