[FirebaseAuth] Add Sign-Out Support for BYO-CIAM in Regionalized GCIP

[srushtisv]

Add R-GCIP Token-Only Sign Out

PR Description:

This PR introduces a new signOut() behavior for Auth instances when they are operating in a Regionalized GCIP (R-GCIP) token-only mode. Previously, the signOut() method would attempt to clear a standard user session, which is not applicable when only an R-GCIP token is present.

Key Changes:

1. R-GCIP Token-Only Sign Out:

   • Modified the Auth.signOut() method to correctly handle R-GCIP token-only sessions. If _currentUser is nil (indicating no standard user session) but an R-GCIP token exists, signOut() will now clear the rGCIPFirebaseTokenLock (setting the token to nil) instead of attempting to clear a non-existent standard user session. This ensures that the R-GCIP token is properly invalidated on sign-out.
   • The exchangeToken method no longer explicitly calls signOut() on a standard user session before updating the R-GCIP token, as the general signOut() method now handles both cases implicitly if a user is present.

2. Auth Instance Management in Sample App (AuthenticationExample):

   • Refactored AppManager in the AuthenticationExample to better manage Auth instances, particularly for regionalized Auth.
   • Introduced cached Auth instances (defaultAppAuth, otherAppAuth, regionalAuthInstance) to avoid repeated creation.
   • The auth() method in AppManager now consistently returns the regionalAuthInstance for regionalized operations, ensuring the correct Auth configuration is always used.
   • Added a standardAuth() method to retrieve standard Auth instances (default or other app) for non-regionalized operations.

3. Sample App Changes:

   • Added a new AuthMenu option: exchangeTokenSignOut.
   • Implemented callExchangeTokenSignOut() in AuthViewController to demonstrate the new R-GCIP token-only sign-out functionality. This allows users to explicitly sign out a regionalized session that only has an R-GCIP token.

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[pashanka]

If the comment is not required please remove it, rest LGTM