[firestore-bigquery-export] Add user to changelog

### [READ] Step 1: Are you in the right place?

Yes

### [REQUIRED] Step 2: Extension name

This feature request is for extension: **\_** (`firestore-bigquery-export`)

### What feature would you like to see?

Add the authed user to change logs.

### How would you use it?

It would be useful for a number of reasons to know who the authenticated user was that made each change to a firestore document.
It would allow us to use the changelog as an audit log, and it would also help us for building reports based on the user that made modifications.