expose additionalUserInfo to firebase user

[GaneshvrBhat]

Issue: Unable to check if the user is a new user or existing user, unless the user reauthenticates after signup.
let's assume we have a Splash screen with FirebaseAuth.instance.authStateChanges().listen((user) {}
Slash screen will route to Login Screen or home Page depending on auth state.

When the user successfully signs in, the listener at the splash screen will get noticed and will route to the home page.
This leaves UserCredential additionalUserInfo obtained during sign in the not usable state unless you save it in local preference or something.

exposing additionalUserInfo to user object helps us to check if the user is new or old and act accordingly.

The other alternative is checking user.metadata.creationTime == user.metadata.lastSignInTime; but this always will be true until user signs out after signup or first login.

Please correct me if I'm wrong