**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 = 'gantt?dag_id=example_bash_operator'
        self.logout()
        self.login(username='dag_faker', password='dag_faker')
        resp = self.client.get(url, follow_redirects=True)
        self.check_content_not_in_response('example_bash_operator', resp)
    def test_success_fail_for_read_only_task_instance_access(self):
        
        self.create_user_and_login(
            username='task_instance_read_user',
            role_name='task_instance_read_role',
            perms=[
                (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG),
                (permissions.ACTION_CAN_READ, permissions.RESOURCE_TASK_INSTANCE),
            ],
        )
        form = dict(
            task_id="run_this_last",
            dag_id="example_bash_operator",
            execution_date=self.default_date,
            upstream="false",
            downstream="false",
            future="false",
            past="false",
        )
        resp = self.client.post('success', data=form)
        self.check_content_not_in_response('Wait a minute', resp, resp_code=302)
    def test_tree_success_for_read_only_role(self):