Event / callback when app is opened via a notification group

## What feature would you like to see?

As described in [react-native-firebase#5716](https://github.com/invertase/react-native-firebase/issues/5716), when the user taps on a notification group, there is no corresponding event / trigger available within the app to know this occurred. It is [hinted](https://github.com/invertase/react-native-firebase/issues/5716#issuecomment-915330980) that this might be a limitation within the Firebase Android SDK itself, rather than an issue with the React Native Firebase library, hence this issue.

## How would you use it?

It looks like there is room within the API for a dedicated method when notifications are opened whilst the app is in the background:

```java
public void onMessagesReceived(@NonNull com.google.firebase.messaging.RemoteMessage[]);
```

It is less clear how this could be approached when a group of notifications are opened whilst the app is quit — I think it is not unreasonable to add a suitable `Extra` to the existing API method:

```java
public final int onStartCommand(android.content.Intent, int, int);
```

Start by reviewing the notification handling APIs around onMessagesReceived and Service.onStartCommand, along with the linked react-native-firebase#5716 discussion. Determine whether notification-group opens can expose a callback or Intent extra when the app is backgrounded or quit. Done means the supported behavior and an implementable API path are established for both app states.

Written by the indexing model from the issue text.