**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
endpoint = "success"
        dag_id = "example_bash_operator"
        form = dict(
            task_id="run_this_last",
            dag_id=dag_id,
            execution_date=self.EXAMPLE_DAG_DEFAULT_DATE,
            upstream="false",
            downstream="false",
            future="false",
            past="false",
            origin="/graph?dag_id=example_bash_operator",
        )
        clear_db_runs()
        resp = self.client.post('success', data=form, follow_redirects=True)
        self.check_content_in_response(f"Cannot make {endpoint}, seem that dag {dag_id} has never run", resp)
    def test_success_flash_hint(self):
        form = dict(
            task_id="run_this_last",
            dag_id="example_bash_operator",
            execution_date=self.EXAMPLE_DAG_DEFAULT_DATE,
            confirmed="true",
            upstream="false",
            downstream="false",
            future="false",
            past="false",
            origin="/graph?dag_id=example_bash_operator",
        )
        resp = self.client.post("success", data=form, follow_redirects=True)
        self.check_content_in_response("Marked success on 1 task instances", resp)
    def test_clear(self):