Make FirebaseApp.client fully public

**Is your feature request related to a problem? Please describe.** Currently, I'm tinkering with `firebase_functions`. These use `FirebaseApp` from `dart_firebase_admin` and expose the app to the consumer. The `FirebaseApp` itself provides core firebase services which use the `package:googleapis_auth` `AuthClient`. However, some of these services (e.g. `storage`) have a trimmed down API, which isn't sufficient for all use cases. For instance, my current requirement is being able to upload a large file to Cloud Storage (several GBs) and fetch the file in ranges. The API provided by default only supports direct data uploads and full data downloads, which isn't feasible for large files. Whereas the `googleapis` package provides a more complete API, which supports streaming uploads. Thus, it would be great if the `AuthClient` was exposed as part of the actual public API, so consumers can use it without having to re-implement the same initialization logic `FirebaseApp` does under the hood, and so we don't have to have separate clients for interacting with the same services. **Describe the solution you'd like** Remove the `@internal` annotation and make `AuthClient get client` part of the public API. https://github.com/firebase/firebase-admin-dart/blob/4872123791ee8a49a17547cc7d4e95b40de886b5/packages/dart_firebase_admin/lib/src/app/firebase_app.dart#L138-L145 **Describe alternatives you've considered** Silence the warning and use the internal API.