Task/workflow state change callback mechanism

[uniqueg]

Is your feature request related to a problem? Please describe.
Identical to elixir-cloud-aai/cwl-WES/issues/57 from WES-ELIXIR.

Describe the solution you'd like
Resolve together with elixir-cloud-aai/cwl-WES/issues/57 from WES-ELIXIR, as both services/apps will be affected.

Describe alternatives you've considered
N/A

Additional context
N/A

[uniqueg]

An implementation should follow this: ga4gh/task-execution-schemas#176

Note that, as a gateway, both a receiver and a sender implementation of this callback would need to be added. Also note that the implementation should be generic enough that the same (or very very similar) module can be added to proWES as well.