Add support for REST hooks to AGOL and AGS services

[MarcHoogerwerf]

I recently stumbled on the concept of REST hooks, a light-weight subscription layer on top of REST API's. It allows a user to subscribe to events happening on resource end-points, without burdening the server by constant polling. It would provide a great way for AGS and AGOL services to communicate with each other and with other external systems.

Clients could subscribe to inserts, updates, deletes on e.g. feature services and based on those events start an action on another service.

The functionality comes close to what Geo-event processor does for near real-time streams, but with REST hooks the concept could also be applied for those services that don't have near real-time updates. Also it would allow for more flexible client side behavior on those events. One client may want to send out an email, the other client may want to update another feature service and yet another one would update a SharePoint list, etc.

It would also allow organisations to collaborate through their feature services in a much more interactive, fun and flexible way.