**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
pass
class TestPluginView(TestBase):
    def test_should_list_plugins_on_page_with_details(self):
        resp = self.client.get('/plugin')
        self.check_content_in_response("test_plugin", resp)
        self.check_content_in_response("Airflow Plugins", resp)
        self.check_content_in_response("source", resp)
        self.check_content_in_response("<em>$PLUGINS_FOLDER/</em>test_plugin.py", resp)
    def test_should_list_entrypoint_plugins_on_page_with_details(self):
        mock_plugin = AirflowPlugin()
        mock_plugin.name = "test_plugin"
        mock_plugin.source = EntryPointSource(
            mock.Mock(), mock.Mock(version='1.0.0', metadata={'name': 'test-entrypoint-testpluginview'})
        )
        with mock_plugin_manager(plugins=[mock_plugin]):
            resp = self.client.get('/plugin')
        self.check_content_in_response("test_plugin", resp)
        self.check_content_in_response("Airflow Plugins", resp)
        self.check_content_in_response("source", resp)
        self.check_content_in_response("<em>test-entrypoint-testpluginview==1.0.0:</em> <Mock id=", resp)
class TestPoolModelView(TestBase):