[firebase_messaging] Return list of topics by SDK

[kayumarie]

Request

Provide a method that would return current list of topics device is subscribed to

await FirebaseMessaging.instance.getTopics()

Reasoning

In order to manage user topics they need to be stored separately in local storage atm. This is a duplicate system, that should be synced properly to ensure user can see topics that they are subscribed to and be able to subscribe/ unsubscribe from certain topics. This makes managing topics fairly complicated, especially taking in consideration network errors. That would be much simpler to receive list of relevant topics from SDK.

[kayumarie]

Having a method to unsubscribe from all topics at once also would be very useful, for the case when user wants to disable all notifications.
await FirebaseMessaging.instance.unsubscribeFromAllTopics()