This commit introduces the UseUserAccessGroup method to the Firebase
Authentication C++ SDK. This method is primarily for iOS and allows
developers to specify a keychain access group for sharing user
credentials across multiple applications from the same developer.

Key changes:
- Added `Auth::UseUserAccessGroup(const char* access_group)` to the
  public API in `firebase/auth.h`.
- Implemented the iOS functionality in `auth_ios.mm`, which calls
  the Objective-C `[FIRAuth useUserAccessGroup:error:]` method.
- Provided stub implementations for Android and desktop platforms where
  this method acts as a no-op, returning `kAuthErrorNone`.
- Added `kAuthFn_UseUserAccessGroup` to the `AuthApiFunction` enum in
  `data.h` for consistency in API function tracking.

This commit implements the `UseUserAccessGroup` method in the Firebase
Authentication C++ SDK.

- Added the public API declaration in `firebase/auth.h` with Doxygen
  comments.
- Implemented the iOS version in `auth_ios.mm` which calls the
  native `[FIRAuth useUserAccessGroup:error:]` method.
- Added a stub implementation for desktop and other platforms in
  `auth_desktop.cc` that returns `kAuthErrorNone` as it's an
  iOS-only feature.