🐛 [firestore-bigquery-export] Add collectiongroup query to generate-schema-views script

### [REQUIRED] Step 2: Extension name

firestore-bigquery-export

### What feature would you like to see?

Add ability to also do collection group queries for generating schemas with gen-schema-views

### How would you use it?

Create schemas also of subcollections

Start by locating the gen-schema-views script in the firestore-bigquery-export extension and read how it currently generates schemas. Determine how collection group queries should be incorporated for subcollections, then verify that schemas are generated for those collections without breaking existing collection handling.

Written by the indexing model from the issue text.