**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
test_dag_id = "example_bash_operator"
        DR = models.DagRun  
        self.session.query(DR).delete()
        self.session.commit()
        response = self.client.post(f'trigger?dag_id={test_dag_id}', data={'conf': '{"a": "b"'})
        self.check_content_in_response('Invalid JSON configuration', response)
        run = self.session.query(DR).filter(DR.dag_id == test_dag_id).first()
        self.assertIsNone(run)
    def test_trigger_dag_form(self):
        test_dag_id = "example_bash_operator"
        resp = self.client.get(f'trigger?dag_id={test_dag_id}')
        self.check_content_in_response(f'Trigger DAG: {test_dag_id}', resp)
    @parameterized.expand(
        [
            ("javascript:alert(1)", "/home"),
            ("http://google.com", "/home"),
            (
                "%2Ftree%3Fdag_id%3Dexample_bash_operator';alert(33)//",
                "/tree?dag_id=example_bash_operator%27&amp;alert%2833%29%2F%2F=",
            ),
            ("%2Ftree%3Fdag_id%3Dexample_bash_operator", "/tree?dag_id=example_bash_operator"),
            ("%2Fgraph%3Fdag_id%3Dexample_bash_operator", "/graph?dag_id=example_bash_operator"),
        ]
    )
    def test_trigger_dag_form_origin_url(self, test_origin, expected_origin):