[FR] Connect to Firebase Emulator (Auth) using custom hostname and port

**Is your feature request related to a problem? Please describe.**
I am trying to setup a testing phase that would run tests on the Emulator by connecting to it. The Emulator is prepared using TestContainers which don't open specific ports, but rather random. Because of this I cannot use the environment variable specified as that blocks me into selecting a hostname and port.

**Describe the solution you'd like**
Make the hostname and port dynamic when initialising FirebaseAuth / FirebaseApp.

**Describe alternatives you've considered**
-

**Additional context**
### Expected usage
```
FirebaseAuth.getInstance(firebaseApp()).useEmulator('localhost', '9099')
```

For example, Firestore has this enabled using `FirestoreOptions.newBuilder().setEmulatorHost()`

まず FirebaseAuth と FirebaseApp の初期化を追跡し、次に既存の FirestoreOptions.setEmulatorHost() アプローチと比較します。FirebaseAuth.getInstance(firebaseApp()).useEmulator('localhost', '9099') がエミュレーター接続用のカスタムホスト名とポートを受け入れ、動的に割り当てられた TestContainers エンドポイントも扱えるようになれば、作業は完了です。

索引モデルが issue の本文から書いたものです。