Add support in the kogito's BPMN Editor for the the Milestone node (case management related).

Background
In BC there exist 2 assets, the one for a process & the one for a case (also the workbench project is case type). On the other hand, for kogito editors, both process / case capabilities will coexist in the same editor. Those capabilities will be added incrementally, by keeping in sync with any runtime related capabiltiies or updates around case management.

This task represents the first milestone for the whole case management achievement (editor's perspective), and it's mostly about adding the support for Milestones (as Ad-hoc/Stage is already supported by the BPMN domain).

Considerations

• Perform Work item definitions loading at startup time
  • This implementation must be kogito specific, not affecting BC editors
  • Perform loading of some pre-defined list of WID's at startup
  • For now only Milestone will be part of the pre-defined startup list
  • This initial WID load my consume first the Resource API, if the definition or service is not available, it will fallback into some internal registry loading of the definition (in the pre-defined startup list)
• The Case Management section (in the properties panel) will not (yet) appear (when Ad-hoc property is set to true). Postponing for further review & work.