fix snapshot attribute in result_set

Author: rekby

ydb/convert.py

@@ -319,6 +319,7 @@ def __init__(self, columns, rows, truncated, snapshot=None):
         self.columns = columns
         self.rows = rows
         self.truncated = truncated
+        self.snapshot = snapshot
 
     @classmethod
     def from_message(cls, message, table_client_settings=None, snapshot=None):