🐛 [firebase_ui_auth] Swift Package Manager (SPM) support

### Is there an existing issue for this?

- [x] I have searched the existing issues and found no duplicates.

### What plugin is this bug for?

Firebase UI OAuth, Firebase UI Auth

### What platform(s) does this bug affect?

macOS, iOS

### List of dependencies used.

flutter pub deps -s list

  desktop_webview_auth: 0.0.15

### Steps to reproduce

* enable SPM support for a Flutter app
* add `firebase_ui_auth` or `firebase_ui_oauth`
* the package `firebase_ui_oauth` pulls in https://pub.dev/packages/desktop_webview_auth which does not support SPM

### Expected Behavior

The package should work with SPM.

### Actual Behavior

The package does not work with SPM.

### Additional Information

I created https://github.com/invertase/flutter_desktop_webview_auth/issues/64 to track SPM support there as well.