API to enable Firestore debug logging

[dconeybe]

Please add a mechanism to FirebaseFirestore to enable/disable debug logging.

This debug logging is incredibly useful when the Firestore SDK team is asked to investigate issues and, currently, it does not appear that there is a way to toggle the debug logging from this Flutter SDK.

In Android, this would involve calling FirebaseFirestore.setLoggingEnabled(true/false) to enable/disable debug logging: https://github.com/firebase/firebase-android-sdk/blob/7ee2c644bf20fde37d2ec52042b38e6e35dbb280/firebase-firestore/src/main/java/com/google/firebase/firestore/FirebaseFirestore.java#L587-L594

In iOS, this would involve calling [FIRFirestore enableLogging:YES/NO] to enable/disable debug logging: https://github.com/firebase/firebase-ios-sdk/blob/5034479ca0c4f32f299677fb0ba1fe4d3e3e20b4/Firestore/Source/Public/FirebaseFirestore/FIRFirestore.h#L254-L255C9

[dconeybe]

Googlers can see b/281831068 for an example where this API would have been useful. There are others as well, which I can look for upon request.

[dconeybe]

Converted to an issue: #10995