**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
follow_redirects=True,
        )
        self.assertEqual(response.status_code, 200)
        response_str = response.data
        if isinstance(response.data, bytes):
            response_str = response_str.decode()
        self.assertEqual(
            json.loads(response_str), {'url': 'https://airflow.apache.org/1.10.5/', 'error': None}
        )
        
        response = self.client.get(
            "{}?dag_id={}&task_id={}&execution_date={}&link_name=google".format(
                self.endpoint, self.dag.dag_id, self.task_3.task_id, self.default_date
            ),
            follow_redirects=True,
        )
        self.assertEqual(response.status_code, 200)
        response_str = response.data
        if isinstance(response.data, bytes):
            response_str = response_str.decode()
        self.assertEqual(json.loads(response_str), {'url': 'https://www.google.com', 'error': None})
class TestDagRunModelView(TestBase):
    @classmethod
    def setUpClass(cls):
        super().setUpClass()
        models.DagBag().get_dag("example_bash_operator").sync_to_db(session=cls.session)
        cls.session.commit()
        cls.clear_table(models.DagRun)
        cls.clear_table(models.TaskInstance)
    def tearDown(self):