feat(engine): provide formKey as property of Task

provide Task.getFormKey() as simple way to get the form key for a task.
Previously an additional service call to the Form Service was necessary.

NOTE: the form key is only populated if taskQuery.initializeFormKeys() is called:

    // if initializeFormKeys() is called
    Task task = taskService.createTaskQuery()
      .processInstanceId(processInstance.getId())
      .initializeFormKeys()
      .singleResult();

    // then the form key is present
    assertEquals("exampleFormKey", task.getFormKey());

    // if initializeFormKeys() is not called
    task = taskService.createTaskQuery()
      .processInstanceId(processInstance.getId())
      .singleResult();

    // then the form key is present
    assertNull(task.getFormKey());

related to #CAM-2568