Convert FIRAuthProvider.m constants into enum #9236

Feature proposal

• Firebase Component: Auth

Description

Currently the file FIRAuthProvider.m contains auth provider ids, as constants.
My request is to replace these with an enum, so it'll be easier to make switch-case block when needed.

It just makes a lot more sense than creating my own enum by manually copying the values from the file or make some non-convenience long and unnecessary else-if.
P.S. You may add as well case for AppleProviderID as "apple.com".

Usage case example

Generate the correct credential for reauthenticate(with: credential) based on Auth.auth().currentUser?.providerData.first?.providerID.