Implement Firebase Auth UseUserAccessGroup for C++ iOS SDK

[jonsimantov]

This commit introduces the UseUserAccessGroup method to the Firebase Authentication C++ SDK.

This method is specific to iOS and allows developers to specify a keychain access group for user data. It calls the underlying Objective-C method [FIRAuth useUserAccessGroup:error:].

On Android and desktop platforms, this method is a no-op stub and returns kAuthErrorNone as per its documented behavior for non-iOS platforms.

Key changes:

• Added Auth::UseUserAccessGroup(const char* access_group) to the public header auth/src/include/firebase/auth.h with Doxygen comments.
• Implemented the iOS-specific logic in auth/src/ios/auth_ios.mm, including error conversion from NSError to AuthError.
• Added stub implementations in auth/src/desktop/auth_desktop.cc and auth/src/android/auth_android.cc returning kAuthErrorNone.

Description

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Testing

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Type of Change

Place an x the applicable box:

• [ ] Bug fix. Add the issue # below if applicable.
• [ ] New feature. A non-breaking change which adds functionality.
• [ ] Other, such as a build process or documentation change.

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Notes

• Bug fixes and feature changes require an update to the Release Notes section of release_build_files/readme.md.
• Read the contribution guidelines CONTRIBUTING.md.
• Changes to the public API require an internal API review. If you'd like to help us make Firebase APIs better, please propose your change in a feature request so that we can discuss it together.