Subscriptions don't support update_relationship

[mcmathews]

Is your feature request related to a problem? Please describe.
I'd like to subscribe to updates on the properties of a relationship. Currently, subscriptions only contain options for subscribing to create_relationship and delete_relationship, but no update. When I update the properties of a relationship, I see the event fired is an update event on the node I updated the relationship through, but the event doesn't contain any information about the change in the relationship's properties. It simply shows the old and new properties of the node (which are the same in both states).

Describe the solution you'd like
I would like to see subscription options for a relationship being updated, not just created and deleted.

Describe alternatives you've considered
No good alternatives. The only thing I've considered is treating node update events with no clear changes as potentially containing an edge property update, but that would be significantly more difficult than just having a relationship update event.

Additional context
None