**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
response = self.client.get(
            "{}?dag_id={}&task_id={}&execution_date={}&link_name=raise_error".format(
                self.endpoint, self.dag.dag_id, self.task.task_id, self.default_date
            ),
            follow_redirects=True,
        )
        self.assertEqual(404, response.status_code)
        response_str = response.data
        if isinstance(response.data, bytes):
            response_str = response_str.decode()
        self.assertEqual(json.loads(response_str), {'url': None, 'error': 'This is an error'})
    def test_extra_links_no_response(self):
        response = self.client.get(
            "{}?dag_id={}&task_id={}&execution_date={}&link_name=no_response".format(
                self.endpoint, self.dag.dag_id, self.task.task_id, self.default_date
            ),
            follow_redirects=True,
        )
        self.assertEqual(response.status_code, 404)
        response_str = response.data
        if isinstance(response.data, bytes):
            response_str = response_str.decode()
        self.assertEqual(json.loads(response_str), {'url': None, 'error': 'No URL found for no_response'})
    def test_operator_extra_link_override_plugin(self):