fix(auth): handle external sign-in callbacks when using scene delegates in sample app

[daymxn]

Per b/522347894,

This fixes the issue within one of the Firebase Auth sample apps, where usage of scene delegates would break the external sign-in flow.

More specifically, within the AuthenticationExample sample app that auth has, the handling of Google sign-in/Facebook sign-in is properly done for the App delegate path. But the scene delegate path does not call either of these methods.

When an app is using scene delegates, certain app delegate methods won't be called. In this case, it's the application:openURL:options: method. The scene delegate already has a corresponding scene:openURLContexts:, but it needs to be updated to call the external sign-in callbacks.

This PR adds those callbacks to the scene delegate.

#no-changelog

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[ncooke3]

Would any changes need to be made for customers? Like docs updates?

[ncooke3]

Also, there are some CI fixes to look into.