A......... [ZBX-8660] Fix API to disallow deleting of discovered entities. Also add validation so that only allowed fields of discovered entities can be updated.
Disallow delete of: 
  - discovered graphs with graph.delete API call,
  - discovered items with item.delete API call,
  - discovered triggers with trigger.delete API call,
  - discovered hosts with host.delete API call.
Add validation when:
  - updating discovered items with item.update API call,
  - updating discovered triggers with trigger.update API call,
  - updating discovered hosts with host.update API call.