Revisit way that tasks are triggered

[helrond]

Is your feature request related to a problem? Please describe.

In our current implementation, tasks can be triggered by configuring a callback request or by passing a post_service_url parameter. This is confusing for implementers and also artificially limits the number of services that can be triggered.

Describe the solution you'd like

• Exclusively use callbacks as a way to trigger services. This will require some refactoring of existing services to rework those that use the POST request method.
• Change model relationships to allow the designation of more than one callback service.