Write policies using Common Expression Language (CEL)

[flavio]

We want to allow users to write policies using CEL. CEL is a programming language that is gaining momentum inside of the Kubernetes world. It's now part of the core Kubernetes, since users can write policies using it. CEL is also easier to write compared to Rego.

Acceptance criteria

• [x] Write initial policy
• [ ] Allow CEL policies to use host capabilities provided by the Kubewarden platform
• [ ] Make the dependency on spec.contextAwareResources for namespaces optional. This allows running context-aware CEL policies out of cluster.
• [ ] Write documentation in CEL policy readme.md
  • [ ] CEL callbacks
• [ ] Write a blog post