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

apps/base/valkey-operator-system/kustomization.yaml

@@ -0,0 +1,6 @@
+---
+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+
+resources:
+  - namespace.yaml

apps/base/valkey-operator-system/namespace.yaml

@@ -0,0 +1,5 @@
+---
+apiVersion: v1
+kind: Namespace
+metadata:
+  name: valkey-operator-system