**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

airflow/www/views.py
error_message = (
                'Given execution date, {}, could not be identified '
                'as a date. Example date format: 2015-11-16T14:34:15+00:00'.format(execution_date)
            )
            response = jsonify({'error': error_message})
            response.status_code = 400
            return response
        task_log_reader = TaskLogReader()
        if not task_log_reader.supports_read:
            return jsonify(
                message="Task log handler does not support read logs.",
                error=True,
                metadata={"end_of_log": True},
            )
        ti = (
            session.query(models.TaskInstance)
            .filter(
                models.TaskInstance.dag_id == dag_id,
                models.TaskInstance.task_id == task_id,
                models.TaskInstance.execution_date == execution_date,
            )
            .first()
        )
        if ti is None:
            return jsonify(
                message="*** Task instance did not exist in the DB\n",
                error=True,
                metadata={"end_of_log": True},
            )
        try: