feat(firestore-bigquery-export): user-facing tooling

[cabljac]

Import script, gen-schema-view, cross-project scripts, and guides are absent from the kit.

Parity ledger: #2974.

[cabljac]

These exist in the repo still, as long as they're still documented, that's all this takes.

[CorieW]

I think this is done https://github.com/firebase/extensions/blob/kits/kits/firestore-bigquery-export/README.md#tooling-that-is-not-included. Closing and checking off on parity issue checklist