**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()
        self.login()
        resp = self.client.post('task_stats', follow_redirects=True)
        self.check_content_not_in_response('example_subdag_operator', resp)
    def test_task_stats_success_for_all_dag_user(self):
        self.create_user_and_login(
            username='task_stats_success_for_all_dag_user',
            role_name='task_stats_success_for_all_dag_user_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),
            ],
        )
        resp = self.client.post('task_stats', follow_redirects=True)
        self.check_content_in_response('example_bash_operator', resp)
        self.check_content_in_response('example_subdag_operator', resp)
    def test_task_stats_success_when_selecting_dags(self):