Client-side Callbacks when a Job executes

[olttwa]

Allow clients to register for a callback when a Job is executed Callback can contain either execution result (success scenario) or exception (failure scenario)

[olttwa]

cc @bsless

[olttwa]

Questions

1. Callbacks make more sense in context of a batch. Should we only reserve them for Batches(#105) or allow individual Jobs to have callbacks as well?
2. Should a callback be (A) a simple function that's provided when creating a Batch or (B) 2-way communication between Application<>Worker over the message broker?
   1. A is easier to implement, won't affect performance of Application and will solve most use-cases
   2. B is more difficult to implement, will affect performance of Application
   3. I suggest we do A. Users wanting B will have to add an API to their Application and call it from the Callback function in Worker