RKE2 support for system-default-registry

Some deployments need to use rke2 system-default-registry option.

Today this is only possible by using this kind of pre-bootstrap command:

(sylva-units values):

cluster:
  additional_commands:
    pre_bootstrap_commands:
      - >-
        echo "system-default-registry: foo.com" >> /etc/rancher/rke2/config.yaml

To cleanly support this, we need:

• support in RKE2 CAPI provider
  • CRDs have the systemDefaultRegistry field (RKE2ControlPlane.spec.agentConfig.systemDefaultRegistry and RKE2ConfigTemplate.spec.template.spec.agentConfig.systemDefaultRegistry ), but the backend isn't implemented yet
  • the full implementation comes with https://github.com/rancher/cluster-api-provider-rke2/pull/739, which needs to land in a release, and then we could use it
• sylva-capi-cluster needs to support this field
  • 👀 this really is what this Issue is about
  • the work on this can start before the support for the functionality lands in RKE2 provider ⬅️