Support namespaceObject in CEL

[Agalin]

Describe the solution you'd like

Gatekeeper's CEL engine seems to lack the top-level namespaceObject defined by Kubernetes docs. It should give access to the request namespace object, including its labels.

Anything else you would like to add:

Didn't check if authorizer object works as expected, others seem to be fine. Note that I've used Gator tests to validate this, didn't deploy to a cluster.

This is already supported in Kyverno (kyverno/kyverno#8070).

Environment:

• Gatekeeper version: 3.18.2
• Kubernetes version: (use kubectl version): N/A