feat(HttpRouteFilter): Allow cross namespace extensionRef

[fgeck]

What type of PR is this?
/kind feature

What this PR does / why we need it:
Allow cross namespace reference for extensionRef in HttpRouteFilter to allow having e.g. Traefik Middlewares defined once in a namespace and not copied over to every namespace where it is used.

Which issue(s) this PR fixes:
Fixes #3903

Does this PR introduce a user-facing change?:

Switch from `LocalObjectReference` to `ObjectReference` for the `ExtensionRef` field of `HTTPRouteFilter` to allow cross namespace reference

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: fgeck / name: Florian Geckeler (e92eca6)

[k8s-ci-robot]

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[fgeck]

This is still WIP. But as is it a breaking change most likely I have to introduce a new API version.

[fgeck]

Open question: Do I have to adapt the tests in pkg/test/cel ?

[fgeck]

no capacity to dive deeper into it