**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
self,
        dag_id,
        task_id,
        origin,
        execution_date,
        confirmed,
        upstream,
        downstream,
        future,
        past,
        state,
    ):
        dag = current_app.dag_bag.get_dag(dag_id)
        task = dag.get_task(task_id)
        task.dag = dag
        latest_execution_date = dag.get_latest_execution_date()
        if not latest_execution_date:
            flash(f"Cannot make {state}, seem that dag {dag_id} has never run", "error")
            return redirect(origin)
        execution_date = timezone.parse(execution_date)
        from airflow.api.common.experimental.mark_tasks import set_state
        if confirmed:
            altered = set_state(
                tasks=[task],
                execution_date=execution_date,
                upstream=upstream,
                downstream=downstream,
                future=future,
                past=past,
                state=state,