Support Cross-Namespace Policy attachment

[guydc]

Description:

@dprotaso writes:

anyone know off hand if an EnvoyExtensionPolicy can match all HTTPRoutes.

I've tried an empty label selector
targetSelectors:

• group: gateway.networking.k8s.io
  kind: HTTPRoute
  matchLabels: {} # Test all HTTPRoutes

[...]

I'm just trying to match all routes right now in the cluster. [...] I'm sidecaring an ext-proc server with the envoy proxy. But repeating the boilerplate config for each namespace is a bit much - do you recommend an alternative. I sorta consider this config as part of the 'gateway' deployment

GW-API currently only support Local Policy References in TargetRef(s).

To support scalable cross-namespace policy attachment, EG can introduce a NamespaceSelector as part of TargetSelectors (A custom EG policy selection mechanism), specifying which namespaces should be considered for policy attachment.

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Any extra documentation required to understand the issue.