Idea Summary
Expose the Activity Definition ID both within APEX_WORKFLOW_ACTIVITIES and as a substitution variable so that configuration on the Activity can be exposed at runtime.

Use Case
Additional configuration may be defined against the Workflow Activity where additional customisation is required.  It is currently not possible to access the Activity Definition within runtime as there is no suitable substitution variable and it is not exposed on the underlying workflow views during runtime.

This is currently a significant omission that is affecting development of workflow applications

Preferred Solution (Optional)
It would assist in adding custom definition against a Workflow Activity and being able to access that at runtime if the Activity Definition ID could be exposed both on the APEX_WORKFLOW_ACTIVITIES view and as a substitution variable APEX$WORKFLOW_ACTIVITY_DEF_ID.