Need support for collectionGroup on Firestore

[joelpoloney]

Docs on it here: https://firebase.google.com/docs/firestore/query-data/queries#collection-group-query.

I added a very rough and ugly version in my fork: https://github.com/siteline/firebase-mock/commit/cfd816cdb3b3a34e984ea35e0ed556fe9a81682d. This isn't perfect by any means and only handled/solved my use-case.

[dmurvihill]

For reference: this feature was added to the Firebase SDK in 6.0.0