**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
"""Triggers DAG Run."""
        dag_id = request.values.get('dag_id')
        origin = get_safe_url(request.values.get('origin'))
        request_conf = request.values.get('conf')
        if request.method == 'GET':
            
            default_conf = ''
            if request_conf:
                default_conf = request_conf
            else:
                try:
                    dag = current_app.dag_bag.get_dag(dag_id)
                    default_conf = json.dumps(dag.params, indent=4)
                except TypeError:
                    flash("Could not pre-populate conf field due to non-JSON-serializable data-types")
            return self.render_template(
                'airflow/trigger.html', dag_id=dag_id, origin=origin, conf=default_conf
            )