feat(engine/rest): add expression to task query

- allows to use expressions for following attributes of a task query:
  assignee, assgineeLike, owner, involvedUser, candidateUser,
  candidateGroup, candidateGroups, createdOn, createdAfter,
  createdBefore, dueDate, dueAfter, dueBefore, followUpDate,
  followUpAfter, followUpBefore
- expressions are also available by the REST API

related to #CAM-2629