**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 = 'tries?days=30&dag_id=example_bash_operator'
        resp = self.client.get(url, follow_redirects=True)
        self.check_content_in_response('example_bash_operator', resp)
    def test_landing_times(self):
        url = 'landing_times?days=30&dag_id=example_bash_operator'
        resp = self.client.get(url, follow_redirects=True)
        self.check_content_in_response('example_bash_operator', resp)
    def test_gantt(self):
        url = 'gantt?dag_id=example_bash_operator'
        resp = self.client.get(url, follow_redirects=True)
        self.check_content_in_response('example_bash_operator', resp)
    def test_code(self):
        url = 'code?dag_id=example_bash_operator'
        resp = self.client.get(url, follow_redirects=True)
        self.check_content_not_in_response('Failed to load file', resp)
        self.check_content_in_response('example_bash_operator', resp)
    def test_code_no_file(self):
        url = 'code?dag_id=example_bash_operator'
        mock_open_patch = mock.mock_open(read_data='')
        mock_open_patch.side_effect = FileNotFoundError
        with mock.patch('builtins.open', mock_open_patch), mock.patch(
            "airflow.models.dagcode.STORE_DAG_CODE", False
        ):
            resp = self.client.get(url, follow_redirects=True)
            self.check_content_in_response('Failed to load file', resp)
            self.check_content_in_response('example_bash_operator', resp)
    @conf_vars({("core", "store_dag_code"): "True"})
    def test_code_from_db(self):