fix: dataset extra import/export (#17740)

* fix: dataset extra import/export

* Update superset/datasets/commands/importers/v1/utils.py

tests/integration_tests/datasets/commands_tests.py

@@ -325,7 +325,10 @@ class TestImportDatasetsCommand(SupersetTestCase):
         assert dataset.template_params == "{}"
         assert dataset.filter_select_enabled
         assert dataset.fetch_values_predicate is None
-        assert dataset.extra == "dttm > sysdate() -10 "
+        assert (
+            dataset.extra
+            == '{"certification": {"certified_by": "Data Platform Team", "details": "This table is the source of truth."}, "warning_markdown": "This is a warning."}'
+        )
 
         # user should be included as one of the owners
         assert dataset.owners == [mock_g.user]