Add cilium to conformance testing

Please add Cilium into knative's testing of the gatewayapi.

[Cilium](https://github.com/cilium/cilium) is a CNI, kube-proxy replacement, sidecar-less service mesh, load balancer, and observability tool that also offers a gateway controller. Cilium 1.15 will support all gateway-api features except for one as seen in their conformance report, https://github.com/kubernetes-sigs/gateway-api/blob/main/conformance/reports/v0.8.1/cilium-cilium.yaml.

Cilium is growing in popularity and leverages some of the most recent Linux features. Greatly appreciate if it could be added and regularly tested against Knative!

Start by examining the repository's existing Gateway API conformance testing setup and compare it with the linked Cilium conformance report at conformance/reports/v0.8.1/cilium-cilium.yaml. The work is complete when Cilium is included in Knative's regular Gateway API conformance testing and its supported features are verified.

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