refactor: handle auto upgrade anonymous improvement

[russellwheatley]

• Handle both credentialAlreadyInUse and emailAlreadyInUse when attempting to link anonymous account with provider (if option set in AuthConfiguration).
• Ensure currentUser is nil when signedOut for immediate feedback in auth service.
• Created an opinionated sign-in handler in AuthPickerView for default Views only which will attempt to link anonymous account with provider and If it fails, it will sign out anonymous user and login with provider. This sign out & login with provider behaviour is not part of AuthService for consumers who don't want it, this means they can get credential from the error thrown (see .accountMergeConflict(context) enum which passes back the credential).
• Also refactored phone auth to be self contained when starting phone auth flow. Removes the need for random setVerificationCode() method.