[FR]: Subscribing to topic in Messaging for multiple projects is not supported?

### Description

In android, we can get different instances for FirebaseMessaging and subscribe to topic for needed FirebaseMessaging. In iOS as I see from documentation only `retrieveFCMTokenForSenderID` and `deleteFCMTokenForSenderID` is available, but there is no way to `subscribeToTopic` for multiple firebase projects. How can we achieve that?

Android: https://firebase.google.com/docs/cloud-messaging/android/topic-messaging#subscribe_the_client_app_to_a_topic

Related issue: (https://github.com/firebase/firebase-ios-sdk/pull/154#issuecomment-323999447)

### API Proposal

Support for `subscribeToTopic` and `unSubscribeToTopic` for multiple firebase projects.

### Firebase Product(s)

Messaging