**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
]
    )
    @action_logging
    def delete(self):
        """Deletes DAG."""
        from airflow.api.common.experimental import delete_dag
        from airflow.exceptions import DagFileExists, DagNotFound
        dag_id = request.values.get('dag_id')
        origin = get_safe_url(request.values.get('origin'))
        try:
            delete_dag.delete_dag(dag_id)
        except DagNotFound:
            flash(f"DAG with id {dag_id} not found. Cannot delete", 'error')
            return redirect(request.referrer)
        except DagFileExists:
            flash(f"Dag id {dag_id} is still in DagBag. Remove the DAG file first.", 'error')
            return redirect(request.referrer)
        flash(f"Deleting DAG with id {dag_id}. May take a couple minutes to fully disappear.")
        
        return redirect(origin)
    @expose('/trigger', methods=['POST', 'GET'])
    @auth.has_access(
        [
            (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG),
            (permissions.ACTION_CAN_CREATE, permissions.RESOURCE_DAG_RUN),
        ]
    )
    @action_logging
    @provide_session
    def trigger(self, session=None):