Add CEL test for `ClientSettingsPolicy` Policy

[shaun-nx]

Overview

As a user of NGF, I would like to add CEL tests for the ClientSettingsPolicies CRD so that I can be confident that the CEL validation works as I expect

UACs

• Add test for the timeout CEL validation
• Add test for the targetRef CEL validation

timeout validation:

x-kubernetes-validations:
    - message: header can only be specified if server is specified
      rule: '!(has(self.header) && !has(self.server))'

targetRef validation:

x-kubernetes-validations:
  - message: 'TargetRef Kind must be one of: Gateway, HTTPRoute, or
      GRPCRoute'
    rule: (self.kind=='Gateway' || self.kind=='HTTPRoute' || self.kind=='GRPCRoute')
  - message: TargetRef Group must be gateway.networking.k8s.io.
    rule: (self.group=='gateway.networking.k8s.io')

[nginx-bot]

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