**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
super().setUp()
        self.logout()
        self.login()
        clear_db_runs()
        self.prepare_dagruns()
    def prepare_dagruns(self):
        self.bash_dag = self.dagbag.get_dag('example_bash_operator')
        self.sub_dag = self.dagbag.get_dag('example_subdag_operator')
        self.xcom_dag = self.dagbag.get_dag('example_xcom')
        self.bash_dagrun = self.bash_dag.create_dagrun(
            run_type=DagRunType.SCHEDULED,
            execution_date=self.EXAMPLE_DAG_DEFAULT_DATE,
            start_date=timezone.utcnow(),
            state=State.RUNNING,
        )
        self.sub_dagrun = self.sub_dag.create_dagrun(
            run_type=DagRunType.SCHEDULED,
            execution_date=self.EXAMPLE_DAG_DEFAULT_DATE,
            start_date=timezone.utcnow(),
            state=State.RUNNING,
        )
        self.xcom_dagrun = self.xcom_dag.create_dagrun(
            run_type=DagRunType.SCHEDULED,
            execution_date=self.EXAMPLE_DAG_DEFAULT_DATE,
            start_date=timezone.utcnow(),
            state=State.RUNNING,
        )
    def test_index(self):