feat: preferRest app option for Firestore

[alexander-fenster]

Fixes #1879 by providing a way to pass preferRest to the Firestore constructor. This is coming from the solution we provided in https://issuetracker.google.com/issues/158014637. The preferRest option might become default in @google-cloud/firestore later (possibly after the next major), but for now, we'd better keep this behind a flag, so the customers asked for a way to pass this flag from firebase-admin.