[firestore-bigquery-export] Make the import script idempotent

[cabljac]

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

Issues filed here should be about a feature request for a specific extension in this repository. To file a feature request that affects multiple extensions or the Firebase Extensions platform, please reach out to
Firebase support directly.

[REQUIRED] Step 2: Extension name

This feature request is for extension: import script

What feature would you like to see?

As mentioned in #1060 it would be nice to have the import script be idempotent