Conversation Notification Support for Firebase Messaging

## What feature would you like to see?

Android 11 has recently introduced [conversation notification](https://developer.android.com/develop/ui/views/notifications/conversations), which allows us to display the user's profile photo on the notification.

To support Conversation Notifications on Android, I would have to use something like [`didReceive()`](https://developer.apple.com/documentation/usernotifications/unnotificationserviceextension/1648229-didreceive) on iOS to modify the notification. However, [`onMessageReceived()`](https://firebase.google.com/docs/cloud-messaging/android/receive#handling_messages) currently doesn't work for background notification message (or notification + data messages).

I also couldn't find an option for Conversation Notification in the REST API or smth (server-side) list: https://firebase.google.com/docs/reference/fcm/rest/v1/projects.messages

I'm not talking about `imageUrl` or `image` option in the REST API ([here](https://firebase.google.com/docs/reference/fcm/rest/v1/projects.messages#notification)): `imageUrl` only seems to work for image attachments, but not for profile photos.

## How would you use it?

I would use it to display profile photo in a notification on Android devices.