**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 = 'task?task_id=runme_0&dag_id=example_bash_operator&execution_date={}'.format(
            self.percent_encode(self.EXAMPLE_DAG_DEFAULT_DATE)
        )
        resp = self.client.get(url, follow_redirects=True)
        self.check_content_in_response('Task Instance Details', resp)
    def test_xcom(self):
        url = 'xcom?task_id=runme_0&dag_id=example_bash_operator&execution_date={}'.format(
            self.percent_encode(self.EXAMPLE_DAG_DEFAULT_DATE)
        )
        resp = self.client.get(url, follow_redirects=True)
        self.check_content_in_response('XCom', resp)
    def test_xcom_list_view_title(self):
        resp = self.client.get('xcom/list', follow_redirects=True)
        self.check_content_in_response('List XComs', resp)
    def test_rendered_template(self):
        url = 'rendered-templates?task_id=runme_0&dag_id=example_bash_operator&execution_date={}'.format(
            self.percent_encode(self.EXAMPLE_DAG_DEFAULT_DATE)
        )
        resp = self.client.get(url, follow_redirects=True)
        self.check_content_in_response('Rendered Template', resp)
    def test_rendered_k8s(self):
        url = 'rendered-k8s?task_id=runme_0&dag_id=example_bash_operator&execution_date={}'.format(
            self.percent_encode(self.EXAMPLE_DAG_DEFAULT_DATE)
        )
        with mock.patch.object(settings, "IS_K8S_OR_K8SCELERY_EXECUTOR", True):
            resp = self.client.get(url, follow_redirects=True)
            self.check_content_in_response('K8s Pod Spec', resp)
    @conf_vars({('core', 'executor'): 'LocalExecutor'})
    def test_rendered_k8s_without_k8s(self):