Deploy cephrgw-operator to cephrgw-system

The cephrgw-operator provisions Ceph RGW (S3) buckets and access keys from
CRDs via the Ceph manager dashboard API. Deploy it as a platform app.

- Add apps/base/cephrgw-system: namespace, ServiceAccount + ClusterRole/
  Binding (manage ceph.unkin.net CRDs, Secrets, leases), and the operator
  Deployment. CRDs are pulled from the operator repo at tag v0.1.0; the
  Deployment sources dashboard credentials from the cephrgw-credentials
  Secret via envFrom and carries the reloader annotation.
- Add apps/overlays/au-syd1/cephrgw-system referencing the base.
- Register apps/overlays/*/cephrgw-system in the platform ApplicationSet.

The platform AppProject already permits *-system namespaces and the
Namespace/ClusterRole/CRD cluster resources, so no project change is needed.
Requires the cephrgw-credentials Secret to be created out-of-band (see the
operator's docs/ceph-setup.md); it is intentionally not managed in GitOps.

apps/base/cephrgw-system/kustomization.yaml

@@ -0,0 +1,11 @@
+---
+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+
+resources:
+  - namespace.yaml
+  # CRDs are pulled from the cephrgw-operator repo at the matching tag rather
+  # than vendored here, so they never drift from the operator.
+  - https://git.unkin.net/unkin/cephrgw-operator/raw/tag/v0.1.0/config/crd/install.yaml
+  - rbac.yaml
+  - deployment.yaml