🚀 [auth] Allow setting custom auth domain for default app

https://github.com/invertase/react-native-firebase/pull/7530 added support for setting the auth domain when calling `initializeApp`, but since we never do that manually for the default app that solution doesn't help in that case. https://github.com/invertase/react-native-firebase/issues/7523#issuecomment-1864458805 claims that you can set it through your `GoogleServices-Info.plist`/`google-services.json` instead. Correct me if I'm wrong, but this doesn't seem to actually be the case. https://cloud.google.com/identity-platform/docs/show-custom-domain#java only shows ways to do it at runtime and I haven't gotten it to work through the config files.

For this reason it would be useful if the auth instance exposed something like a `setCustomAuthDomain` function that took in the auth domain and just proxied to those native functions to set it.