[crashlytics] add method to specify firebase app

## What feature would you like to see?

Add `FirebaseCrashlytics instance(FirebaseApp app)` method, so that not the default application is used.

## How would you use it?

We have a firebase project for each backend stage in our application. Since we are dealing with sensitive user data, not everyone can just look at all the projects. We want to sent crashlytics to a specific firebase project, depending on which stage of the backend the user select. But currently only the default application is used.

Since we're building a flutter application, this also have to be implemented in the ios SDK and the flutterfire/firebase_crashlytics plugin. (see also https://github.com/firebase/flutterfire/issues/9109).