[DmnServiceTask] dmn table key as expression

**Problem description:**
Cmmn/Bpmn dmnService task does not define dmnTable as an expression. Dmn reference must be a reference to existing dmn model in the application. Partially related issue is:
https://forum.flowable.org/t/modeler-dmn-reference-lost-after-bpmn-import/8598
In the case when dmn table is not recognized in export/import of the individual process/case model the information is lost because reference does not exist. In this case we can store the information in dmn table key.

**Solution proposal:**
Adding a new property `Decision table key` for dmnService tasks in modeler with export/import.

**Alternative:**
Add expression to dmn model popup.