[cloud_firestore] _CodecUtility isn't testable/mockable.

[sensuikan1973]

For example, mocking arrayUnion with DocumentReference is impossible.

It's because _CodecUtility sees DocumentReference._deleagte.
https://github.com/FirebaseExtended/flutterfire/blob/77b27d042e766b09ba12a680697b6be38f78564d/packages/cloud_firestore/cloud_firestore/lib/src/utils/codec_utility.dart#L46-L47
delegate is private. So, we can't mock that.

Ofcourse, delegate should be private.
So, I wonder if the solution is to fix _CodecUtility testable.

[Ehesp]

Related; #4563