[Conformance] Gateway should validate the returned endpoints from Inference pool

[rikatz]

As discussed with @robscott :

The inference pool (and the endpoint picker) may return a diverse set of endpoints, and one desired behavior is that the Gateway validates that the returned endpoints are part of a well known admin configuration.

As an example:

• Gateway Admin may want to allow just endpoints that belong to the cluster (eg.: inside the Pod CIDR, or part of a created endpoint/endpointslice)
• Gateway Admin may want to allow just endpoints that belong to the same namespace of the InferencePool used by the HTTPRoute
• Gateway Admin may want to allow well known endpoints like external pool names/IPs

This way, a conformance test that does the following steps is desired:

• Deploy a generic InferencePool/EndpointPicker that returns the endpoints based on generic requests (eg.: some header)
• Deploy a generic echo backend
• Given the following requests, the actions should be taken:
  • A request to the gateway, passing the endpoint of an existing backend on the same namespace should be accepted and forwarded to the right endpoint by the gateway
  • A request to the gateway, passing the endpoint of an existing backend on a different namespace should be denied / return an error (unless there is a referencegrant for a inferencepool on a different namespace)
  • A request to the gateway, passing the endpoint of an external resource should be accepted depending on ImplementationSpecific behavior (to be defined)

[robscott]

/triage accepted
/area conformance

[k8s-ci-robot]

@robscott: The label(s) area/conformance cannot be applied, because the repository doesn't have them.

In response to this:

/triage accepted
/area conformance

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