The Firebase app to initialize Auth for.
Optional_deps: DependenciesOptional firebase-js-sdk dependency bag.
The optional deps argument exists for firebase-js-sdk API parity. React Native Firebase
ignores persistence, popup redirect resolver, and error-map dependencies because native
iOS/Android SDKs manage auth state and do not support the web-only persistence stack.
initializeAuth() returns the same shared Auth instance as getAuth.
This function allows more control over the Auth instance than getAuth().