Support for Performer, HumanPerformer, PotentialOwner

* Add `Performer` interface and `PerformerImpl` class to support `performer` BPMN element
* Add `HumanPerformer` interface and `HumanPerformerImpl` class to support `humanPerformer` BPMN element
* Add `PotentialOwner` interface and `PotentialOwnerImpl` class to support potentialOwner BPMN element