Link Multiple Auth Providers to a Firebase Auth Account

[wimbledon]

Link Multiple Auth Providers to a Firebase Auth Account Firebase Auth Documentation https://firebase.google.com/docs/auth/ios/account-linking

Given that Firebase Auth supports multiple auth provider linking, expose this feature / API through FirebaseAuthUI.

[wimbledon]

@morganchen12 I would like to request this feature as the 1000th issue.

[wimbledon]

Related issue: #744