🐛 [cloud_firestore] Add source option for realtime snapshots() listener

[michalkubizna]

The one-time get call has a GetOptions(source: Source. cache) option. Add the same option to the snapshots() listener. Use case: A real-time listener is not possible to implement with pagination. Without a real-time listener, the current user's updates are not reflected in the UI. Setting up a listener that would only listen to updates from the cache would allow for pagination while still including updates that come from the local device.