[flutterfire_ui] allow more customization on SignInView

The auth widgets are great, huge time saver. My only request is a simple Divider widget between the third-party auth buttons and the email/password boxes just to give them a little more separation. Maybe a label on those to say "Sign in with email". Right now the buttons and form elements look confusingly related.

_Originally posted by @johnsheehan in https://github.com/FirebaseExtended/flutterfire/discussions/6978#discussioncomment-1802674_