**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 = 'duration?days=30&dag_id=example_bash_operator'
        self.create_user_and_login(
            username='duration_success_user',
            role_name='duration_success_role',
            perms=[
                (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG),
                (permissions.ACTION_CAN_READ, permissions.RESOURCE_TASK_INSTANCE),
                (permissions.ACTION_CAN_READ, permissions.RESOURCE_WEBSITE),
            ],
        )
        resp = self.client.get(url, follow_redirects=True)
        self.check_content_in_response('example_bash_operator', resp)
    def test_duration_failure(self):