FR: Add API for checking subscription status of a given topic

In order to show users a list of subscribed topics (eg UI with the ability to remove active subscriptions) we have to do one of two things today:

- implement a shadow storage of active subscriptions (in Firebase DB or elsewhere), with all the potential for sync-misshaps
- implement the [instance ID](https://developers.google.com/instance-id/reference/server#get_information_about_app_instances) API and have the app query our server, which then queries Googles servers, resulting in more latency, server maintenance/dev overhead

I sorely miss a `Messaging.messaging().isSubscribed(to: topic)` like API in the FCM SDK for checking whether a topic is currently subscribed to by the current user