refactor(firestore-bigquery-export): importer script code

[yamankatby]

Remove unnecessary duplicate code:

https://github.com/firebase/extensions/blob/8efd4e18afdb62218e1422deee1d60c89df9ebb9/firestore-bigquery-export/scripts/import/src/index.ts#L146

https://github.com/firebase/extensions/blob/8efd4e18afdb62218e1422deee1d60c89df9ebb9/firestore-bigquery-export/scripts/import/src/config.ts#L68

fixes #911

The batchSize param was passed without parsing it to integer https://github.com/firebase/extensions/blob/8efd4e18afdb62218e1422deee1d60c89df9ebb9/firestore-bigquery-export/scripts/import/src/run.ts#L38

Now it is parsed directly in the parseConfig method

fixes #962

When I use multi-threads, I have to answer the same questions twice.)