**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 = 'dag_details?dag_id=example_subdag_operator.section-1'
        resp = self.client.get(url, follow_redirects=True)
        self.check_content_in_response('DAG Details', resp)
    def test_graph(self):
        url = 'graph?dag_id=example_bash_operator'
        resp = self.client.get(url, follow_redirects=True)
        self.check_content_in_response('runme_1', resp)
    def test_last_dagruns(self):
        resp = self.client.post('last_dagruns', follow_redirects=True)
        self.check_content_in_response('example_bash_operator', resp)
    def test_last_dagruns_success_when_selecting_dags(self):
        resp = self.client.post(
            'last_dagruns', data={'dag_ids': ['example_subdag_operator']}, follow_redirects=True
        )
        self.assertEqual(resp.status_code, 200)
        stats = json.loads(resp.data.decode('utf-8'))
        self.assertNotIn('example_bash_operator', stats)
        self.assertIn('example_subdag_operator', stats)
        
        resp = self.client.post(
            'last_dagruns',
            data={'dag_ids': ['example_subdag_operator', 'example_bash_operator']},
            follow_redirects=True,
        )
        stats = json.loads(resp.data.decode('utf-8'))
        self.assertIn('example_bash_operator', stats)
        self.assertIn('example_subdag_operator', stats)
        self.check_content_not_in_response('example_xcom', resp)
    def test_tree(self):