Polymorphism for workflows and actions

The idea is to treat actions and workflows uniformly when defining tasks. From a task perspective there shouldn't be any differences between an action and a workflow. As long as they take defined set of parameters and implement required logic they can be called. So in this sense we can talk about polymorphic behavior regarding tasks and workflows.

Think of different options to implement this:
- call subworkflow via the same REST API (recursive API call)
- drop a message into a workflow queue

This requirement may leads to the need to define workflows themselves differently. First of all, they need to have explicitly declared input parameters and output in a similar way to what actions have.