Only create dynamic_active_clusters for Services and Upstreams that are referenced by routes

[DuncanDoyle]

kgateway version

1.18.4

Is your feature request related to a problem? Please describe.

Currently, Envoy dynamic_active_clusters are created for every Upstream and every Kubernetes Services that is watched by the control plane. Often a lot of these clusters are actually not referenced by routes, but are still part of the xDS snapshot and the the proxy's configuration. This has performance impact on translation, xds sync, and the proxy's memory consumption. Furthermore, useless metrics are created for all these dynamic_active_clusters as they are not being used by any route configured on the proxy

Describe the solution you'd like

We should only create dynamic_active_clusters for Upstreams and Services in an Envoy proxy's configuration when that given proxy contains routes that reference these Upstreams and Services.

Describe alternatives you've considered

In Gloo Edge, we have the disableKubernetesDestinations: true setting that prevents dynamic_active_clusters to be automatically created for Kubernetes Services. Since Gloo Gateway/KGateway 1.18, this feature no longer works for Kubernetes Gateway API based gateway-proxies.

Additional Context

n.a.