workflows: new API in engine and model
- Updates the BibWorkflowObject with new API functions to add widgets to trigger an holding pen action.
- Adds a function to BibWorkflowObject to add task results using a newly created class WorkflowTaskResult.
- Changes the data preview in Holding Pen back to using the previously added get_formatted_data function.
- Widgets and tasks now use these new functions.
- Enriches sample workflow to showcase more tasks.
Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>