feat(cloudnativePG): support topologySpreadConstraints

A HA `Cluster` is only actually highly available if its instances end up in different failure domains. Pod
anti-affinity alone cannot express that: it only keeps the instances off the same node, not off the same zone,
and `cloudnativePG.affinity` has no way to encode a skew budget. Without `topologySpreadConstraints` a three
instance cluster can legitimately be scheduled into a single availability zone, where a zone outage takes down
the primary and both replicas at once.

Both `affinity` and `topologySpreadConstraints` are rendered only when they are set. The CloudNativePG API
declares them as `omitempty`, so an empty `{}` or `[]` in the manifest would show up as a permanent diff against
the state the operator writes back.

Tolerations deliberately do not get a dedicated value. CloudNativePG has no `spec.tolerations`, it nests them
below `spec.affinity.tolerations`, so they are configured through `cloudnativePG.affinity` like `nodeSelector`
and the anti-affinity settings. A separate top-level value would only have to be folded back into the same place
and would silently conflict with a tolerations list set via `cloudnativePG.affinity`.

Co-authored-by: Copilot <copilot@github.com>

templates/cluster.yaml

@@ -12,8 +12,10 @@ metadata:
   name: {{ include "gitea.cluster.cloudnativePG.name" . }}
   namespace: {{ .Values.namespace | default .Release.Namespace }}
 spec:
+  {{- with .Values.cloudnativePG.affinity }}
   affinity:
-    {{- toYaml .Values.cloudnativePG.affinity | nindent 4 }}
+    {{- toYaml . | nindent 4 }}
+  {{- end }}
   bootstrap:
     initdb:
       database: {{ .Values.cloudnativePG.credentials.database | quote }}
@@ -35,6 +37,10 @@ spec:
     {{- with .Values.cloudnativePG.storage.storageClass }}
     storageClass: {{ . | quote }}
     {{- end }}
+  {{- with .Values.cloudnativePG.topologySpreadConstraints }}
+  topologySpreadConstraints:
+    {{- toYaml . | nindent 4 }}
+  {{- end }}
   {{- if .Values.cloudnativePG.walStorage.enabled }}
   walStorage:
     size: {{ .Values.cloudnativePG.walStorage.size | quote }}