Task/581 v1 context entities attributes value 4ops

### DEscription
New set of convops:
GET/PUT/POST/DELETE /v1/contextEntities/type/{type}/id/{id}/attributes/{attr_name}/{id_value}

Maybe this checking (i.e. the length of responses returned by mongoBackend is no longer than the 1 that the conv op response support) should be a common pattern if we want to avoid ambiguity until we implement #588.

However, how are we doing in order conv ops with the same problem beyong this particular DELETE?