[Feature Request][Cloud Messaging] List subscribed FCM Topic

## What feature would you like to see?

I would like to be able to list all currently subscribed FCM topic of the user.

## How would you use it?

For example, if we have authentication and FCM topics based on it. So when the user log out, we need to unsubscribe certain topics programmatically.

It would help a lot if the SDK could provide list of subscribed FCM topics