FR: add offline (IndexedDB) flag to queries

As it is today, [offline data access](https://firebase.google.com/docs/firestore/manage-data/enable-offline) can only be enabled for all cases. Imagine a case with two user types:

* **Users** don't have access to sensitive information, and can enable IndexedDB right away.
* **Administrators** have access to sensitive information, and they need to confirm if they are on a trusted device before enabling offline access.

Now, assume that the ratio between Users and Administrators is 100x (which is common: there are way more Users than Administrators). This leaves with two options:

* **Ask every User and Administrator if they are on a trusted device**, which is unnecessary friction to Users (100x in ratio) and carries performance loss if they decide not to turn it on
* **Enable offline by default and present an opt-out to Administrators**, which is insecure by default (note: this does not seem possible with the current API, where offline access cannot be disabled)

**The Request**

It would be nice to have an `offline` flag associated to the query, assuming that sensitive information is not associated to the connection as a whole, but to the specific collections accessed in each part of the application.