**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.logout()
        username = 'task_stats_success_when_selecting_dags_user'
        self.create_user_and_login(
            username=username,
            role_name='task_stats_success_when_selecting_dags_role',
            perms=[
                (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG),
                (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG_RUN),
                (permissions.ACTION_CAN_READ, permissions.RESOURCE_TASK_INSTANCE),
                (permissions.ACTION_CAN_READ, permissions.RESOURCE_WEBSITE),
            ],
        )
        self.login(username=username, password=username)
        resp = self.client.post(
            'task_stats', 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(
            'task_stats',
            data={'dag_ids': ['example_subdag_operator', 'example_bash_operator']},
            follow_redirects=True,
        )