**Description**
In the trigger page, there is a text area to enter (optional) conf json. It would be great if a sample JSON can be provided programatically while defining DAG

**Use case / motivation**

This will improve usability of the UI that triggers a DAG

tests/www/test_views.py
test_dag_id = "example_bash_operator"
        resp = self.client.get(f'trigger?dag_id={test_dag_id}&origin={test_origin}')
        self.check_content_in_response(
            '<button type="button" class="btn" onclick="location.href = \'{}\'; return false">'.format(
                expected_origin
            ),
            resp,
        )
    @parameterized.expand(
        [
            (None, {"example_key": "example_value"}),
            ({"other": "test_data", "key": 12}, {"other": "test_data", "key": 12}),
        ]
    )
    def test_trigger_dag_params_conf(self, request_conf, expected_conf):