[#67] Use tuple of header and signature in object head response

Object.Head method returns either full header or short header.
Since it can't return anything else, signature must be paired
with full header in explicit tuple message.

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

object/service.proto

@@ -239,23 +239,29 @@ message HeadRequest {
   neo.fs.v2.session.RequestVerificationHeader verify_header = 3;
 }
 
+// Tuple of full object header and signature of object ID.
+message HeaderWithSignature {
+  // Full object header
+  Header header = 1;
+
+  // 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 = 2;
+}
+
 // Head response
 message HeadResponse {
   // Response body
   message Body {
     // Carries the requested object header or it's part
     oneof head{
-      // Full object header
-      Header header = 1;
+      // Full object header with object ID signature
+      Header HeaderWithSignature = 1;
 
       // 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.