message: Add if exists into rename statements
diff: @@ -71,6 +71,7 @@ unique_ptr<AlterStatement> Transformer::TransformRename(duckdb_libpgquery::PGNod
  throw NotImplementedException("Schema element not supported yet!");
  }
  D_ASSERT(info);
  + info->if_exists = stmt->missing_ok;
  auto result = make_unique<AlterStatement>();
  result->info = move(info);
  