**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(username='dag_faker', password='dag_faker')
        url = 'task?task_id=runme_0&dag_id=example_bash_operator&execution_date={}'.format(
            self.percent_encode(self.default_date)
        )
        resp = self.client.get(url, follow_redirects=True)
        self.check_content_not_in_response('Task Instance Details', resp)
    def test_task_success_for_all_dag_user(self):
        self.create_user_and_login(
            username='task_success_for_all_dag_user',
            role_name='task_success_for_all_dag_user_role',
            perms=[
                (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG),
                (permissions.ACTION_CAN_READ, permissions.RESOURCE_TASK_INSTANCE),
                (permissions.ACTION_CAN_READ, permissions.RESOURCE_WEBSITE),
            ],
        )
        url = 'task?task_id=runme_0&dag_id=example_bash_operator&execution_date={}'.format(
            self.percent_encode(self.default_date)
        )
        resp = self.client.get(url, follow_redirects=True)
        self.check_content_in_response('Task Instance Details', resp)
    def test_xcom_success(self):