**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
EXAMPLE_DAG_DEFAULT_DATE = dates.days_ago(2)
    @classmethod
    def setUpClass(cls):
        super().setUpClass()
        models.DagBag(include_examples=True, read_dags_from_db=False).sync_to_db()
        dagbag = models.DagBag(include_examples=True, read_dags_from_db=True)
        cls.bash_dag = dagbag.get_dag('example_bash_operator')
        cls.sub_dag = dagbag.get_dag('example_subdag_operator')
        cls.xcom_dag = dagbag.get_dag('example_xcom')
    def setUp(self):
        super().setUp()
        self.logout()
        self.login()
        clear_db_runs()
        self.prepare_dagruns()
    def prepare_dagruns(self):