**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
resp = self.client.get('/', follow_redirects=True)
        self.assertIn('/trigger?dag_id=example_bash_operator', resp.data.decode('utf-8'))
        self.assertIn("return confirmDeleteDag(this, 'example_bash_operator')", resp.data.decode('utf-8'))
    @pytest.mark.quarantined
    def test_trigger_dag_button(self):
        test_dag_id = "example_bash_operator"
        DR = models.DagRun  
        self.session.query(DR).delete()
        self.session.commit()
        self.client.post(f'trigger?dag_id={test_dag_id}')
        run = self.session.query(DR).filter(DR.dag_id == test_dag_id).first()
        self.assertIsNotNone(run)
        self.assertIn(DagRunType.MANUAL, run.run_id)
        self.assertEqual(run.run_type, DagRunType.MANUAL)
    @pytest.mark.quarantined
    def test_trigger_dag_conf(self):
        test_dag_id = "example_bash_operator"
        conf_dict = {'string': 'Hello, World!'}
        DR = models.DagRun  
        self.session.query(DR).delete()
        self.session.commit()
        self.client.post(f'trigger?dag_id={test_dag_id}', data={'conf': json.dumps(conf_dict)})
        run = self.session.query(DR).filter(DR.dag_id == test_dag_id).first()
        self.assertIsNotNone(run)
        self.assertIn(DagRunType.MANUAL, run.run_id)
        self.assertEqual(run.run_type, DagRunType.MANUAL)
        self.assertEqual(run.conf, conf_dict)
    def test_trigger_dag_conf_malformed(self):