🐛 [firestore-bigquery-export] Configure Firestore events that trigger the export

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

I think so. It is a change to the firestore-bigquery-export extension.

### [REQUIRED] Step 2: Extension name

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

### What feature would you like to see?

I would like to be able to mark certain Firestore events to be ignored by the export function. For example, being able to disable exports for Deletes operations, and not transferring those to BigQuery. Don't get me wrong, I still think all operations should be enabled by default, but having that extra configuration would help.

### How would you use it?

In my case, and I suppose that other too, I have a TTL (time-to-live) configured on the exporting collection, since it increases a lot over time and I don't need all the data after X days, but I would like to maintain it on BigQuery for more time.
When deleting these documents, I don't need to notify BigQuery of these changes, which would decrease resources needed and costs (logs, storage, bandwidth, ...).