[2.8.1 Backport] CBG-1244 - Add a new rev option for document_changed event handler

The document_changed event handler is fired for every mutation of a document, even if that mutation was for a non-winning revision.

There are use-cases where having a webhook fired only in the event of a winning revision change makes sense.

Proposal:

Add an EventHandler options map, to provide a way of specifying generic options for each handler.

The document_changed webhook should use an opt-in option, that prevents the webhook from firing unless it's due to a new winning revision.

Test scenarios

Include positive and negative testing for events using new option.