**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
super().setUp()
        clear_db_runs()
        self.prepare_dagruns()
        self.logout()
        self.appbuilder.sm.sync_roles()
        self.add_permission_for_role()
    def login(self, username='dag_tester', password='dag_test'):
        dag_tester_role = self.appbuilder.sm.add_role('dag_acl_tester')
        if username == 'dag_tester' and not self.appbuilder.sm.find_user(username='dag_tester'):
            self.appbuilder.sm.add_user(
                username='dag_tester',
                first_name='dag_test',
                last_name='dag_test',
                email='dag_test@fab.org',
                role=dag_tester_role,
                password='dag_test',
            )
        
        dag_no_role = self.appbuilder.sm.add_role('dag_acl_faker')
        if username == 'dag_faker' and not self.appbuilder.sm.find_user(username='dag_faker'):
            self.appbuilder.sm.add_user(
                username='dag_faker',