feat(cockpit): extension point for toolbar actions

- New extension points:

  * 'cockpit.processInstance.action' for action buttons in process
    instance view
  * 'cockpit.processDefinition.action' for action buttons in process
    definition view

- Renamed extension point:

  'cockpit.processInstance.instanceDetails' is renamed to
  'cockpit.processInstance.view' for consistency with process definition
  view.

  Compatibility with existing extension point is maintained; providers
  will be loaded but sorted at the end of the tab list.

related ot #CAM-1542