Implement PolicyGroups RFC

[viccuad]

Implement RFC PolicyGroups.

Acceptance criteria

• [x] Add PolicyGroups to policy-server. This includes changing policy-evaluator. https://github.com/kubewarden/policy-server/pull/814
• [x] #833
  • Define the CRDs Go types.
  • Implement the reconciliation of the new CRDs, make sure we can reuse as much as possible of the sub-reconciler we already have in place.
• [x] #857
• [x] #859
• [x] https://github.com/kubewarden/kubewarden-controller/issues/872
• [x] https://github.com/kubewarden/kubewarden-controller/issues/876
• [x] https://github.com/kubewarden/audit-scanner/issues/366
• [x] https://github.com/kubewarden/kubewarden-controller/issues/842
• [x] #858
• [x] Docs:
  • [x] https://github.com/kubewarden/kubewarden-controller/issues/857
  • [x] Add explanation page.
  • [x] Add HowTo page
• [ ] Blogpost.
• [ ] Add PolicyGroups to kwctl (kwctl run, kwctl scaffold) [Optional]

[flavio]

This has been done now