Task/581 v1 context entities attributes 4ops

### Description
Four new conv ops:
GET/PUT/POST/DELETE /v1/contextEntities/type/TYPE/id/ID/attributes/ATTR_NAME

I would suggest leave it as it is (i.e. with the original code and the unit test working as they are now). Note that the goal of #581 is to add entity type to the URL of some existing conv op, but not to fix issues in the existing conv op (such fixing is matter of a different issue). I understand that he problem you describe already is in "'DELETE /v1/contextEntitie/id/ID/attribute/ATTR_NAME".

However, I don't mind to keep the FIXME P9 and if 0 where it is now (maybe adjusting the text of the coment based on the above paragraph), to be taken into account in the future.l