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`.

This commit incorporates feedback from the code review:

- Updated Doxygen comments in `auth.h` for `UseUserAccessGroup` to refer to Firebase iOS SDK documentation for keychain details, removing the extensive inline explanation.
- Corrected the error code in `auth_ios.mm` from the non-existent `kAuthErrorUninitialized` to `kAuthErrorFailure` when `auth_data_` is null.
- Added a new integration test `TestUseUserAccessGroupDoesNotCrash` in `auth/integration_test/src/integration_test.cc`. This test calls the function with a sample group and nullptr, checking for `kAuthErrorNone` or `kAuthErrorKeychainError` on iOS (to ensure no crash even if keychain isn't fully set up in test env) and `kAuthErrorNone` on other platforms, primarily ensuring the calls do not crash.

This commit incorporates feedback from the code review regarding the
integration test for `UseUserAccessGroup`:

- Simplified the `TestUseUserAccessGroupDoesNotCrash` test in
  `auth/integration_test/src/integration_test.cc`.
- Removed platform-specific checks (`#if TARGET_OS_IPHONE`) and
  associated `EXPECT_THAT` calls.
- Removed `LogDebug` messages from the test.
- Both calls to `UseUserAccessGroup` now uniformly expect
  `firebase::auth::kAuthErrorNone`. This aligns with the stub behavior
  on non-iOS platforms and simplifies the test as requested by the
  reviewer, focusing on ensuring the calls do not crash and stubs
  return the expected no-op error code.

This commit incorporates further feedback on the integration test for
`UseUserAccessGroup`:

- Removed the verbose explanatory comment block from
  `TestUseUserAccessGroupDoesNotCrash` in
  `auth/integration_test/src/integration_test.cc`.
- Reinstated platform-specific error checking within the test:
    - On iOS, the test now uses `EXPECT_THAT` to allow either
      `kAuthErrorNone` or `kAuthErrorKeychainError`. This handles
      potential keychain configuration issues in test environments while
      ensuring the call does not crash.
    - On other platforms (Android/Desktop), the test continues to expect
      `kAuthErrorNone`, verifying the stub implementation.