[firestore-bigquery-export] create_time and update_time fields not exported

This feature request is for extension: **firestore-bigquery-export**

### What feature would you like to see?

Firestore sets create_time and update_time by default on every document. ([docs](https://cloud.google.com/firestore/docs/reference/rpc/google.firestore.v1beta1#document)). However, these do not appear in the `..._raw_latest` view or `..._raw_changelog`table.

There is a `timestamp` column, which is similar. However, afaict this is not expected or guarenteed to match the firestore `created_time` value, nor is is easy to get separate created and updated timestamps in the `_latest` table.

### How would you use it?

Auditing in-order execution of document creation

The request concerns the firestore-bigquery-export extension and its _raw_latest and _raw_changelog views. Start by tracing how Firestore document fields are exported to BigQuery, using the linked Firestore Document reference. Done means separate create_time and update_time values are available in both views, with the relevant behavior covered by tests.

Written by the indexing model from the issue text.