allow the change altStatName for backend

Currently, we build cluster name with the rule like httproute/default/backend/rule/0, this's fine for mostly of the cases.

The problem is that when we want to know what the exract hostname/backend name in metrics or tracing spane name.

Maybe we should provide a way to customize it directly beside ClusterStatName in EnvoyProxy.

xref: https://envoyproxy.slack.com/archives/C03E6NHLESV/p1762944475362659

cc @envoyproxy/gateway-maintainers