Ability to set/unset project service trigger events via API

Zendesk: https://gitlab.zendesk.com/agent/tickets/41703

As far as I can tell, it's not currently possible to toggle trigger events on project services via the API. These events are things like push_events, merge_request_events, etc. I thought it was possible by sending push_events=false, etc as parameters, but it's not. The reason is the API::Helpers#service_attributes does not show these events as valid fields. It looks like it could be adapted fairly easily.

@timothyandrew Looks like you recently did some work in this area. What do you think? Is this fairly reasonable to implement?

cc/ @gl-product