Get webhook delivery stats

curl --request GET \
  --url https://cloud-api.phala.com/api/v1/workspace/webhooks/{webhook_id}/stats

{
  "total": 123,
  "success_count": 123,
  "failed_count": 123,
  "success_rate": 123,
  "avg_duration_ms": 123,
  "max_duration_ms": 123,
  "last_delivery_at": "2023-11-07T05:31:56Z"
}