feat(engine-rest/engine): Extend TaskRestService and TaskService

GET /task/{id}/comment
    returns all comments for task

GET /task/{id}/comment/{commentId}
    returns particular comment for task

POST /task/{id}/comment/create
    add comment for task and return comment object with link for the
    request

- extend engine task service to get particular comment for task id and
  comment id
- change addComment method from return value 'void' to 'Comment'
- add test cases

related to #CAM-2134, #CAM-2135, #CAM-2148