[#26] refs: Define ObjectID message

This commit defines ObjectID message in refs package. In the future, this
structure can be ported into messages in place of the object identifier
field to maintain format uniformity. Also, storing the identifier in a
dedicated message will allow, if necessary, to expand it with additional
information without losing backward compatibility within one version of the
API.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

refs/types.proto

@@ -17,3 +17,9 @@ message Address {
     // CID is container identifier
     bytes CID      = 2[(gogoproto.customtype) = "CID",      (gogoproto.nullable) = false];
 }
+
+// ObjectID groups information about the NeoFS object identifier.
+message ObjectID {
+    // Value carries the object identifier in a binary format.
+    bytes Value = 1;
+}