feat(firestore-bigquery-exports) add an option to skip BigQuery init steps

[gjanvier]

This feature would solve this issue: https://github.com/firebase/extensions/issues/2468

When running backfill command with @firebaseextensions/fs-bq-import-collection, there is no reason to alter existing BigQuery table or view, when everything is working fine.

This MR adds an option --skip-init to skip those steps (alterning clustering / partitionning of the raw_changelog, creating the latest view).

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