FR: Add method to get all subscribed topics

[tfonfara]

Environment

• Xcode version: Version 9.4.1 (9F2000)
• Firebase SDK version: 5.2.0
• Firebase Component: Messaging
• Component version: 3.0.2

Feature Proposal

Currently it's not possible to unregister from all subscribed topics (I know there's a workaround to create a new instance id in order to do that) or to get a list of subscribed topics to add/remove subscriptions on that basis.
There's already an API to do that: https://developers.google.com/instance-id/reference/server#get_information_about_app_instances
It would be great if it would be possible to integrate that API (at least the topics part) into the SDK.

[morganchen12]

Similar to #225, but not quite identical.

[FViscardi]

This functionality would be great.

Handling subscribed topics right now is a bit of an unnecessary hassle, as I have to keep track of it myself via firestore or shared preferences.

[dunyakirkali]

@morganchen12 Any update?

[alzalabany]

Any update?

[halabe15]

You can do it through GET request. Check this Stack overflow post.

[dunyakirkali]

Thnx for the link @halabe15. I'm afraid, what you've shared is more of an hack than an actual feature for the iOS SDK.

I believe that the intention of this issue is to get this feature added to the SDK.

Nevertheless thanx a lot!