feat(engine-rest/engine): Add attachment methods to TaskRestService

GET /task/{id}/attachment
    returns all attachments for a task

GET /task/{id}/attachment/{attachmentId}
    returns particular attachment for a task

GET /task/{id}/attachment/{attachmentId}/data
    returns content for an attachment of a task

DELETE /task/{id}/attachment/{attachmentId}
    delete a particular attachment of a task

POST /task/{id}/attachment
    add attachment to a task and return attachment object with link for
    the request

- add new command to get task attachment for task id and attachment id in
  with usage in engine task service
- add new command to get task attachment content for task id and
  attachment id with usage in engine task service
- add  new command to delete task attachment for task id and attachment id
  with usage in engine task service

- add testcases for TaskRestService and TaskService

related to #CAM-2136, #CAM-2137, #CAM-2138, #CAM-2139, #CAM-2140