[#66] object: Add object_id signature to Head response

Object ID signature is needed to check Header authenticity.

Signed-off-by: Stanislav Bogatyrev <stanislav@nspcc.ru>

object/service.proto

@@ -250,6 +250,12 @@ message HeadResponse {
 
       // Short object header
       ShortHeader short_header = 2;
+
+      // Signed object_id to verify full header's authenticity through following steps:
+      // 1. Calculate SHA-256 of marshalled Headers structure.
+      // 2. Check if the resulting hash matched ObjectID
+      // 3. Check if ObjectID's signature in signature field is correct.
+      neo.fs.v2.refs.Signature signature = 3;
     }
   }
   // Body of head object response message.