**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
TI_ENDPOINT = '/taskreschedule/list/?_flt_0_execution_date={}'
    def test_start_date_filter(self):
        resp = self.client.get(self.TI_ENDPOINT.format(self.percent_encode('2018-10-09 22:44:31')))
        
        
        
        self.check_content_in_response('List Task Reschedule', resp)
class TestRenderedView(TestBase):
    def setUp(self):
        self.default_date = datetime(2020, 3, 1)
        self.dag = DAG(
            "testdag",
            start_date=self.default_date,
            user_defined_filters={"hello": lambda name: f'Hello {name}'},
            user_defined_macros={"fullname": lambda fname, lname: f'{fname} {lname}'},
        )
        self.task1 = BashOperator(task_id='task1', bash_command='{{ task_instance_key_str }}', dag=self.dag)
        self.task2 = BashOperator(
            task_id='task2', bash_command='echo {{ fullname("Apache", "Airflow") | hello }}', dag=self.dag
        )
        SerializedDagModel.write_dag(self.dag)
        with create_session() as session:
            session.query(RTIF).delete()
        self.app.dag_bag = mock.MagicMock(**{'get_dag.return_value': self.dag})
        super().setUp()
    def tearDown(self) -> None: