* [auth-swift] Port 'FIRAuthStoredUserManager'
* Migrate usage to temporary ported type
* Delete 'FIRAuthStoredUserManager.[hm]'
* Rename temporary type to have original type name
* Remove TODOs and fix docs
* Attempt at merging own ported swift code to official repo
* Fix: networkError was accidentally marked as internal.
* Added Copyright notices
* Moved base64 encoding extension. Reformatted a few todos
* Converted a bit of MultiFactor
* Added VerifyPhoneNumberRequest/Response and AuthOperationType