feat: add imageRegistryCredentials handler

[dkoshkin]

This is big boy 😬
Adds a handler to set image registry credentials on the Cluster using Kubelet image credential provider
and dynamic credential provider.

      - name: clusterConfig
        value:
          imageRegistryCredentials:
            url: https://my-registry.io
            secret: my-registry-credentials

This handler also expects a Secret with keys username and password to already exist. Alternatively we can simplify this and expect an existing Secret as CAPI SecretFileSource that the handler doesn't need to read/write the Secret. But that didn't seem right as the format of the Secret would need to be very specific to the dynamic credential provider project.

---

Also verified by create a docker cluster and checking the KubeadmConfig

[dkoshkin]

Tested with a private image in a Docker cluster:

Events:
  Type     Reason                  Age                From               Message
  ----     ------                  ----               ----               -------
  Normal   Scheduled               42s                default-scheduler  Successfully assigned default/nginx to docker-quick-start-md-0-krcqf-25k4v-r5k84
  Warning  FailedCreatePodSandBox  41s                kubelet            Failed to create pod sandbox: rpc error: code = Unknown desc = failed to setup network for sandbox "9b29e435643db3670bc4185f354708dc719157d6f1e2de121f86a992ebac1ba2": plugin type="calico" failed (add): stat /var/lib/calico/nodename: no such file or directory: check that the calico/node container is running and has mounted /var/lib/calico/
  Normal   SandboxChanged          27s (x2 over 41s)  kubelet            Pod sandbox changed, it will be killed and re-created.
  Normal   Pulling                 26s                kubelet            Pulling image "dkoshkin/nginx:stable"
  Normal   Pulled                  9s                 kubelet            Successfully pulled image "dkoshkin/nginx:stable" in 9.280283022s (16.86162273s including waiting)
  Normal   Created                 9s                 kubelet            Created container nginx
  Normal   Started                 9s                 kubelet            Started container nginx

[dkoshkin]

Thank you @jimmidyson for the thorough review and the fixups!

[jimmidyson]

@dkoshkin Can we remove the do not merge label or are you still testing stuff out?

[dkoshkin]

🚢 thanks again @jimmidyson for the fixups, if you're happy with this lets merge