Allow apps to make Play Integrity API Standard Requests (as opposed to the default Classic Requests)

The Play Integrity API documentation indicates that we should be using standard requests for the majority of our use cases. However, after an exhaustive search, I don't believe the Flutter firebase_app_check plugin supports this feature. Indeed, our Play Integrity quotas dashboard in Google Play indicates that all of our token requests are made using Classic Requests. The App Check plugin should support this feature.