Add cephrgw-operator k8s auth role + dashboard-credentials read policy

The cephrgw-operator reads its Ceph dashboard credentials from Vault via VSO
and needs a Kubernetes auth role bound to its ServiceAccount plus a policy
granting read on the KV path.

- Add config/auth_kubernetes_role/k8s/au/syd1/cephrgw-operator.yaml binding
  ServiceAccount cephrgw-operator in namespace cephrgw-system (audience vault).
- Add policies/kv/service/cephrgw/dashboard-credentials/read.yaml granting
  read on kv/data/service/cephrgw/dashboard-credentials to that role via the
  k8s/au/syd1 auth mount.

config/auth_kubernetes_role/k8s/au/syd1/cephrgw-operator.yaml

@@ -0,0 +1,7 @@
+bound_service_account_names:
+  - cephrgw-operator
+bound_service_account_namespaces:
+  - cephrgw-system
+token_ttl: 600
+token_max_ttl: 600
+audience: vault

policies/kv/service/cephrgw/dashboard-credentials/read.yaml

@@ -0,0 +1,10 @@
+# Allow the cephrgw-operator to read its Ceph dashboard credentials
+---
+rules:
+  - path: "kv/data/service/cephrgw/dashboard-credentials"
+    capabilities:
+      - read
+
+auth:
+  k8s/au/syd1:
+    - cephrgw-operator