Support Firestore SDK SnapshotOptions

[jorroll]

When subscribing to Firestore data without using rxfire, the Firestore SDK (at least v9) supports optional SnapshotOptions when getting data from a snapshot. E.g.

// Currently, the only SnapshotOption is `serverTimestamps`.
snap.data({ serverTimestamps: "estimate" });

When using docData() or collectionData(), in addition to allowing the user to provide an optional idField, I'd like to be able to provide SnapshotOptions and, specifically, the serverTimestamps option so that returned documents have an "estimate" timestamp value rather than null.

If it were possible, I might recommend upstreaming the idField option to the main Firebase SDK repo so that "idField" is added to SnapshotOptions and becomes officially supported. At that point, rxfire could simply pass all the SnapshotOptions provided to docData or collectionData internally to snap.data(options).