**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
url = 'rendered-k8s?task_id=runme_0&dag_id=example_bash_operator&execution_date={}'.format(
            self.percent_encode(self.EXAMPLE_DAG_DEFAULT_DATE)
        )
        resp = self.client.get(url, follow_redirects=True)
        self.assertEqual(404, resp.status_code)
    def test_blocked(self):
        url = 'blocked'
        resp = self.client.post(url, follow_redirects=True)
        self.assertEqual(200, resp.status_code)
    def test_dag_stats(self):
        resp = self.client.post('dag_stats', follow_redirects=True)
        self.assertEqual(resp.status_code, 200)
    def test_task_stats(self):
        resp = self.client.post('task_stats', follow_redirects=True)
        self.assertEqual(resp.status_code, 200)
        self.assertEqual(set(list(resp.json.items())[0][1][0].keys()), {'state', 'count'})
    @conf_vars({("webserver", "show_recent_stats_for_completed_runs"): "False"})
    def test_task_stats_only_noncompleted(self):
        resp = self.client.post('task_stats', follow_redirects=True)
        self.assertEqual(resp.status_code, 200)
    def test_dag_details(self):
        url = 'dag_details?dag_id=example_bash_operator'
        resp = self.client.get(url, follow_redirects=True)
        self.check_content_in_response('DAG Details', resp)
    @parameterized.expand(["graph", "tree", "dag_details"])
    def test_view_uses_existing_dagbag(self, endpoint):