Allow disabling automatic dismissal upon completion

[ipodishima]

Hi

In my app, the way I built my authentication process is as follow:

• display an onboarding screen modally if not yet seen
• after onboarding is seen, display the authentication screen
• dismiss when done

1/ I don't want the authentication screen to be displayed modally on top of the onboarding modal screen
2/ When the onboarding is already seen, having the auth screen modally would require me some specific tweakings for the presentation.

Also, I'm using an AuthenticatorViewController which displays the AuthController for firebase (for abstracting the auth layer in case I need a switch to something else). This is using a view model, which in fine holds the delegate for FUIAuth.
When I present the screen, I'm able to display it while pushing it (authUI.authViewController().viewControllers[0]).
However, when the signin is done, auth controller dismisses the screen (deallocating my view model and my delegate) and then call upon completion the delegate (too late)

Here is my suggested fix https://github.com/firebase/FirebaseUI-iOS/compare/main...ipodishima:feature/allow_non_automatic_dismisses?expand=1