[flutter_core] Add config file name options to Firebase.initializeApp

[aliak00]

Would it be possible to allow an option to specify which plist/json file to use at runtime?

I know how to do it with flavors and build configs, etc,

But I would like to at runtime override which firebase configuration file is used for purposes such as debugging

Something like:

FirebaseApp defaultApp = await Firebase.initializeApp(
  fileOptions: FileOptions({
    iOS: 'GoogleService-Info-file1.plist',
    android: 'custom-configuration.json'
  })
);