feat: add 'extra' column to saved_queries GET (apache#13662)

This column contains schedule information when the feature flag
`SCHEDULED_QUERIES` is enabled. Adding it here makes it available
through the REST API.

superset/queries/saved_queries/api.py

@@ -98,6 +98,7 @@ class SavedQueryRestApi(BaseSupersetModelRestApi):
         "sql_tables",
         "rows",
         "last_run_delta_humanized",
+        "extra",
     ]
     add_columns = ["db_id", "description", "label", "schema", "sql"]
     edit_columns = add_columns