grafana-system: deploy grafana-operator

Adds the grafana-operator (grafana.integreatly.org CRDs + controller) so
Grafana and its dashboards/datasources can be managed declaratively as
CRs in a follow-up PR. Sits in the platform project like the other
operators (vm-system, cnpg-system).

Changes:
- Add grafana-system namespace + grafana-operator helm chart v5.24.0
  (watches all namespaces).
- Render CRDs inline (crds.immutable: false) so ArgoCD installs/manages
  the 13 grafana.integreatly.org CRDs instead of the skipped helm crds/
  subchart.
- Register apps/overlays/*/grafana-system in the platform ApplicationSet.

apps/base/grafana-system/namespace.yaml

@@ -0,0 +1,7 @@
+---
+apiVersion: v1
+kind: Namespace
+metadata:
+  labels:
+    app.kubernetes.io/name: grafana-system
+  name: grafana-system