**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
@parameterized.expand(
        [
            ("", "/home"),
            ("http://google.com", "/home"),
            (
                "http://localhost:8080/trigger?dag_id=test_dag&origin=%2Ftree%3Fdag_id%test_dag';alert(33)//",
                "http://localhost:8080/trigger?dag_id=test_dag&origin=%2Ftree%3F"
                "dag_id%25test_dag%27&alert%2833%29%2F%2F=",
            ),
            (
                "http://localhost:8080/trigger?dag_id=test_dag&origin=%2Ftree%3Fdag_id%test_dag",
                "http://localhost:8080/trigger?dag_id=test_dag&origin=%2Ftree%3Fdag_id%25test_dag",
            ),
        ]
    )
    @mock.patch("airflow.www.views.url_for")
    def test_get_safe_url(self, test_url, expected_url, mock_url_for):
        mock_url_for.return_value = "/home"
        with self.app.test_request_context(base_url="http://localhost:8080"):
            self.assertEqual(get_safe_url(test_url), expected_url)