[cloud_firestore] Support serverTimestampBehavior in cloud_firestore

[jangruenwaldt]

An issue with the same name was already closed, but only FieldValue.serverTimestamp() was implemented. However, what I need is ServerTimestampBehavior = ESTIMATE.

https://firebase.google.com/docs/reference/android/com/google/firebase/firestore/DocumentSnapshot.ServerTimestampBehavior

[ Otherwise I can not cache my firestore data properly, as I filter by minimum timestamp, and if the device is offline, new offline added data will not show up. Thanks! ]