Install valkey-operator via helm

Adds the valkey-operator (v0.5.0) as a platform app so a later change can
replace the self-managed redis Deployment with a Valkey CR.

- Add apps/base/valkey-operator-system (namespace)
- Add apps/overlays/au-syd1/valkey-operator-system pulling the valkey-operator
  0.5.0 chart from the artifactapi virtual helm repo, with includeCRDs and the
  operator image routed through the artifactapi ghcr proxy
- Register the app path in the platform ApplicationSet

argocd/applicationsets/platform.yaml

@@ -43,6 +43,7 @@ spec:
       - path: apps/overlays/*/reloader-system
       - path: apps/overlays/*/reposync
       - path: apps/overlays/*/traefik-system
+      - path: apps/overlays/*/valkey-operator-system
       - path: apps/overlays/*/vm-system
       - path: apps/overlays/*/vpa-system
       - path: apps/overlays/*/vault