**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
dag_orm = session.query(models.DagModel).filter(models.DagModel.dag_id == dag_id).first()
        if not dag_orm:
            flash(f"Cannot find dag {dag_id}")
            return redirect(origin)
        execution_date = timezone.utcnow()
        dr = DagRun.find(dag_id=dag_id, execution_date=execution_date, run_type=DagRunType.MANUAL)
        if dr:
            flash(f"This run_id {dr.run_id} already exists")  
            return redirect(origin)
        run_conf = {}
        if request_conf:
            try:
                run_conf = json.loads(request_conf)
            except json.decoder.JSONDecodeError:
                flash("Invalid JSON configuration", "error")
                return self.render_template(
                    'airflow/trigger.html', dag_id=dag_id, origin=origin, conf=request_conf
                )
        dag = current_app.dag_bag.get_dag(dag_id)
        dag.create_dagrun(
            run_type=DagRunType.MANUAL,
            execution_date=execution_date,
            state=State.RUNNING,
            conf=run_conf,
            external_trigger=True,
            dag_hash=current_app.dag_bag.dags_hash.get(dag_id),
        )
        flash(f"Triggered {dag_id}, it should start any moment now.")
        return redirect(origin)
    def _clear_dag_tis(