Don't allow duplicate TargetRefs for NGF Policies

[kate-osborn]

It's possible to create an ObservabilityPolicy or UpstreamSettingsPolicy with duplicate targetRefs. This is not a valid configuration and should be prevented.

A/C:

• Add CEL validation to ObservabilityPolicy and UpstreamSettingsPolicy to ensure that targetRefs are unique.

Dev Notes:

• see Add CEL for BackendTLSPolicy targetRefs kubernetes-sigs/gateway-api#3496 for an example

[github-actions]

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[mpstefan]

Hey @kate-osborn

What happens when the user does put in a duplicate target ref? Any idea on the effort for a fix?