Add convert anonymous account to permanent account via Google linking

[DiplomaticRobot]

Add the ability to convert an anonymous FirebaseAuth user to a permanent Google account using account linking APIs. Use linkWithCredential with a GoogleAuthProvider credential.

Link of reference: https://firebase.google.com/docs/auth/android/anonymous-auth#convert-an-anonymous-account-to-a-permanent-account

[DiplomaticRobot]

@syntaxerror247 please assign me this issue as I will be working on this starting tomorrow