Add option to collect user name during email registration

[lesnitsky]

Discussed in https://github.com/firebase/FirebaseUI-Flutter/discussions/69

^{Originally posted by bakua August 23, 2023} Hi there! I've been enjoying Firebase UI Auth, but I've noticed something. While social logins easily get display names, email sign-ups don't. Would it be possible to add an option to collect user names during email registration? It would really help keep a consistent experience for users. Thank you so much for considering this!