Job execution callback

[xBlaz3kx]

Implement job execution callback.

Users should be able to specify a callback endpoint (HTTP or AMQP-based), which will get invoked after a job has executed. The information included in the callback request body should be:

1. Job ID
2. Execution time
3. Status (successful/unsuccessful)
4. Original response from the job's invoked endpoint.

The execution callback endpoint is optional, and can be specified both in the create and update requests. If the callback endpoint is specified, it must be validated (URL only).