Implement callback support

[mwvaughn]

Though Abaco allows us to build event-driven systems, we can't learn information about its state without polling. This puts load on the system and introduces latency to downstream consumers in the form of minimum polling intervals (especially if they implement exponential backoff and the desired event occurs right after the last poll).

It would be advantageous if Abaco could post to callbacks when events such as the following occur.

• actor created
• actor updated
• actor scaled
• actor shared
• actor deleted
• execution start
• execution end
• execution failed
• nonce created
• nonce deleted

At minimum, https POST with a non-customizable payload should be supported. Support for one or more authn/authz HTTP headers would be useful. URL parameters should be allowed in callbackURLs and any user-specified ordering should be respected.