[cloud_firestore] Make create_time and update_time available on Documents

[eduardothiesen]

It would be great if we could access the create_time and update_time that are available in the firestore documents as per the documentation: https://cloud.google.com/firestore/docs/reference/rpc/google.firestore.v1#google.firestore.v1.Document

As I am aware of right now it isn't possible to do so.
I know this is easily fixed by creating my own createTime field on my document collections, but for older projects that we initially didn't see the need for a field like that, we can't now generate accurate data based on it.

Thanks in advance.