Add Request Response Size to Cluster Grafana dashboard

Enhance grafana dashboard to add Request response sizes Panel ([suggested here](https://github.com/envoyproxy/gateway/pull/4314#issuecomment-2377783704)). Request response sizes histogram metrics can be enabled by setting `enabledRequestResponseSizeMetric` to true in `ProxyMetric` once #4314 is merged.

Changes needs to be made in `charts/gateway-addons-helm/dashboards/envoy-clusters.json`

I have one concern about adding this: first time users will see this panel empty as this stat will be disabled by default. If this behaviour is fine, we can add.