**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
self.login(username='all_dag_user', password='all_dag_user')
        resp = self.client.get('dagmodel/autocomplete?query=example_bash', follow_redirects=False)
        self.check_content_in_response('example_bash_operator', resp)
        self.check_content_not_in_response('example_subdag_operator', resp)
    def test_dag_stats_success(self):
        self.create_user_and_login(
            username='dag_stats_success_user',
            role_name='dag_stats_success_role',
            perms=[
                (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG),
                (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG_RUN),
                (permissions.ACTION_CAN_READ, permissions.RESOURCE_WEBSITE),
            ],
        )
        resp = self.client.post('dag_stats', follow_redirects=True)
        self.check_content_in_response('example_bash_operator', resp)
        self.assertEqual(set(list(resp.json.items())[0][1][0].keys()), {'state', 'count'})
    def test_dag_stats_failure(self):