Support Firestore SDK `SnapshotOptions`

When subscribing to Firestore data without using rxfire, the Firestore SDK (at least v9) supports [optional `SnapshotOptions`](https://firebase.google.com/docs/reference/node/firebase.firestore.SnapshotOptions) when getting data from a snapshot. E.g.

```ts
// 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)`.