**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
resp = self.client.post(
            'blocked', data={'dag_ids': ['example_subdag_operator']}, follow_redirects=True
        )
        self.assertEqual(resp.status_code, 200)
        blocked_dags = {blocked['dag_id'] for blocked in json.loads(resp.data.decode('utf-8'))}
        self.assertNotIn('example_bash_operator', blocked_dags)
        self.assertIn('example_subdag_operator', blocked_dags)
        
        resp = self.client.post(
            'blocked',
            data={'dag_ids': ['example_subdag_operator', 'example_bash_operator']},
            follow_redirects=True,
        )
        blocked_dags = {blocked['dag_id'] for blocked in json.loads(resp.data.decode('utf-8'))}
        self.assertIn('example_bash_operator', blocked_dags)
        self.assertIn('example_subdag_operator', blocked_dags)
        self.check_content_not_in_response('example_xcom', resp)
    def test_failed_success(self):