**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
self.create_user_and_login(
            username='role_permission_associate_user',
            role_name='role_permission_associate_role',
            perms=[
                (permissions.ACTION_CAN_EDIT, 'DAG:example_bash_operator'),
                (permissions.ACTION_CAN_READ, 'DAG:example_bash_operator'),
            ],
        )
        test_role = self.appbuilder.sm.find_role('role_permission_associate_role')
        perms = {str(perm) for perm in test_role.permissions}
        self.assertIn('can edit on DAG:example_bash_operator', perms)
        self.assertIn('can read on DAG:example_bash_operator', perms)
    def test_index_success(self):