**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().setUpClass()
        dagbag = models.DagBag(include_examples=True)
        DAG.bulk_write_to_db(dagbag.dags.values())
        for username in ['all_dag_user', 'dag_read_only', 'dag_faker', 'dag_tester']:
            user = cls.appbuilder.sm.find_user(username=username)
            if user:
                cls.appbuilder.sm.del_register_user(user)
    def prepare_dagruns(self):
        dagbag = models.DagBag(include_examples=True, read_dags_from_db=True)
        self.bash_dag = dagbag.get_dag("example_bash_operator")
        self.sub_dag = dagbag.get_dag("example_subdag_operator")
        self.bash_dagrun = self.bash_dag.create_dagrun(
            run_type=DagRunType.SCHEDULED,
            execution_date=self.default_date,
            start_date=timezone.utcnow(),
            state=State.RUNNING,
        )
        self.sub_dagrun = self.sub_dag.create_dagrun(
            run_type=DagRunType.SCHEDULED,
            execution_date=self.default_date,
            start_date=timezone.utcnow(),
            state=State.RUNNING,
        )
    def setUp(self):