**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
def setUp(self):
        super().setUp()
        self.connection = {
            'conn_id': 'test_conn',
            'conn_type': 'http',
            'description': 'description',
            'host': 'localhost',
            'port': 8080,
            'username': 'root',
            'password': 'admin',
        }
    def tearDown(self):
        self.clear_table(Connection)
        super().tearDown()
    def test_create_connection(self):
        resp = self.client.post('/connection/add', data=self.connection, follow_redirects=True)
        self.check_content_in_response('Added Row', resp)
    def test_prefill_form_null_extra(self):
        mock_form = mock.Mock()
        mock_form.data = {"conn_id": "test", "extra": None}
        cmv = ConnectionModelView()
        cmv.prefill_form(form=mock_form, pk=1)
class TestVariableModelView(TestBase):
    def setUp(self):
        super().setUp()
        self.variable = {'key': 'test_key', 'val': 'text_val', 'is_encrypted': True}
    def tearDown(self):