🐛 [Stream Firestore to BigQuery] import script: allow to set transformFunction option

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[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: firebase/firestore-bigquery-export

What feature would you like to see?

The extension allows to define a transformFunction option. The import script should allow to set the same option

How would you use it?

execute the import script with the same transformFunction used in the extension