Feature Request: Expose host capabilities to CEL

[fabriziosestito]

We want to allow the user to use Kubwarden's host capabilities while writing CEL expressions. By doing this we can extend the CEL language with Kubwarden's specific features, while keeping the compatibility with ValidatingAdmissionPolicies.

Acceptance Criteria

• [x] #14
• [x] #31
• [x] #16
• [x] #15
• [x] ~Add e2e tests~ we decided to not write e2e tests as the libraries can be easily tested with a mockery generated mock
• [x] Update documentation: ensure the README explains to end users how to use the host capabilities we expose
• [x] Kubewarden docs: mention CEL support, link to this repository for more information about the specific language extensions we have done with this EPIC
  • Links to https://kubernetes.io/docs/reference/access-authn-authz/validating-admission-policy/
  • Ported examples from https://kubernetes.io/docs/reference/access-authn-authz/validating-admission-policy/
  • Ingress policy example
  • Sigstore policy example
• [x] Move the mockery-generated mock in policy-sdk-go and release and consume v0.8.1

[viccuad]

Closing as completed, docs are in next.