[Feature request w/ contribution] Provide a way to access the Firebase instance id

[ghost]

Hi 👋

I need a way to access the Firebase instance id in Flutter, and am willing to contribute the plugin implementation for it. What is the best place to add this API? The firebase_messaging plugin already has a deleteInstanceID method, so maybe getInstanceID belongs there? https://github.com/FirebaseExtended/flutterfire/blob/4d7b2bdd7fbdee65551754968b140653ea16128f/packages/firebase_messaging/lib/firebase_messaging.dart#L166-L173

[Nathan-Heller]

This would also be very useful for the in_app_messaging feature, since the InstanceID is required to test sending an in-app message via the console.