feat: implement dttm column configuration through db extra config (#9444)

* Implement table mutator and examples how to set date defaults

Fix tests

* Fix flaky test

Co-authored-by: bogdan kyryliuk <bogdankyryliuk@dropbox.com>

tests/dict_import_export_tests.py

@@ -280,6 +280,9 @@ class DictImportExportTests(SupersetTestCase):
         )
 
     def test_export_datasource_ui_cli(self):
+        # TODO(bkyryliuk): find fake db is leaking from
+        self.delete_fake_db()
+
         cli_export = export_to_dict(
             session=db.session,
             recursive=True,