feat(rest-api): Extend task dou for attachment

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

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