object/replicate: Add signature to the replication RPC

Object signing now may be requested to ensure a replication copy was received.
Closes #299.

Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>

object/service.proto

@@ -721,10 +721,20 @@ message ReplicateRequest {
 
   // Signature of `object.object_id.value` field.
   neo.fs.v2.refs.Signature signature = 2;
+
+  // Optional flag that requires server side to attach signature of just
+  // replicated object to ensure it has been received correctly. Signature
+  // must be calculated with an exposed to network map public key that
+  // corresponds to the object receiver.
+  bool create = 3;
 }
 
 // Replicate RPC response
 message ReplicateResponse {
   // Operation execution status with one of the enumerated codes.
   neo.fs.v2.status.Status status = 1;
+
+  // Signature of replicated object. Must be attached if a request was
+  // made with `create` flag set.
+  bytes object_signature = 2;
 }