Create Action processes like plugin registrations

[mkholt]

Is your feature request related to a problem? Please describe.
We currently support automatic registration of plugins and plugin steps.
However, creating an action / workflow codeactivity required manual work.

Describe the solution you'd like
Two features would greatly improve the functionality:

1. Register CodeActivities as Actions, handling input and output parameters
2. Support custom messages when registering plugins, which can be used to call directly without setting up an Action.