Perf: cpu spikes

[zirain]

Recently I looks into the cpu usage of control plane.

main-pprof.gz

There serval methods we need to looks into deep:

1. resources.GetEndpointSlicesForBackend
2. resources.GetService
3. (*gatewayAPIReconciler).processBackendRefs

I think #7535 will help on (1) and (2).
But we need find a way to optimize the cost of list endpointslices by service.

cc @envoyproxy/gateway-maintainers