**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
dag = self.dagbag.get_dag('test_tree_view')
        dag.create_dagrun(
            run_type=DagRunType.SCHEDULED,
            execution_date=self.EXAMPLE_DAG_DEFAULT_DATE,
            start_date=timezone.utcnow(),
            state=State.RUNNING,
        )
        url = 'dag_details?dag_id=test_tree_view'
        resp = self.client.get(url, follow_redirects=True)
        params = {'dag_id': 'test_tree_view', 'origin': '/tree?dag_id=test_tree_view'}
        href = "/trigger?{}".format(html.escape(urllib.parse.urlencode(params)))
        self.check_content_in_response(href, resp)
    def test_dag_details_trigger_origin_graph_view(self):
        dag = self.dagbag.get_dag('test_graph_view')
        dag.create_dagrun(
            run_type=DagRunType.SCHEDULED,
            execution_date=self.EXAMPLE_DAG_DEFAULT_DATE,
            start_date=timezone.utcnow(),
            state=State.RUNNING,
        )
        url = 'dag_details?dag_id=test_graph_view'
        resp = self.client.get(url, follow_redirects=True)
        params = {'dag_id': 'test_graph_view', 'origin': '/graph?dag_id=test_graph_view'}
        href = "/trigger?{}".format(html.escape(urllib.parse.urlencode(params)))
        self.check_content_in_response(href, resp)
    def test_dag_details_subdag(self):