OptionalaccessOptionalfullOnly retained for apple.com credentials with at least one non-empty name field,
and only forwarded to the native SDK on iOS, where it is passed to
FIROAuthProvider.appleCredentialWithIDToken:rawNonce:fullName: so Firebase can store it as
User.displayName while creating the account. The Firebase Android SDK has no
equivalent credential-level API; on Android (and Web) this field is ignored and the display
name must be set with updateProfile after sign-in.
OptionalidOptionalraw
The user's full name, as shared by Sign in with Apple on the user's first authorization.