Add DriftIgnoreRules support to flux diff kustomization

Signed-off-by: Dipti Pai <diptipai89@outlook.com>
Assisted-by: GitHub Copilot/Claude Opus 4.7

cmd/flux/testdata/diff-kustomization/diff-with-drift-ignore.golden

@@ -0,0 +1,14 @@
+► Deployment/default/podinfo created
+► HorizontalPodAutoscaler/default/podinfo created
+► Service/default/podinfo drifted
+
+metadata
++ one map entry added:
+  labels:
+    kustomize.toolkit.fluxcd.io/name: podinfo
+    kustomize.toolkit.fluxcd.io/namespace:
+
+► Secret/default/docker-secret created
+► Secret/default/secret-basic-auth-stringdata created
+► Secret/default/podinfo-token-77t89m9b67 created
+► Secret/default/db-user-pass-bkbd782d2c created

cmd/flux/testdata/diff-kustomization/drifted-service-no-labels.yaml

@@ -0,0 +1,18 @@
+apiVersion: v1
+kind: Service
+metadata:
+  name: podinfo
+  namespace: default
+spec:
+  type: ClusterIP
+  selector:
+    app: podinfo
+  ports:
+    - name: http
+      port: 9899
+      protocol: TCP
+      targetPort: http
+    - port: 9999
+      targetPort: grpc
+      protocol: TCP
+      name: grpc

cmd/flux/testdata/diff-kustomization/flux-kustomization-drift-ignore.yaml

@@ -0,0 +1,19 @@
+---
+apiVersion: kustomize.toolkit.fluxcd.io/v1
+kind: Kustomization
+metadata:
+  name: podinfo
+spec:
+  interval: 5m0s
+  path: ./kustomize
+  force: true
+  prune: true
+  sourceRef:
+    kind: GitRepository
+    name: podinfo
+  targetNamespace: default
+  ignore:
+    - paths:
+        - "/spec/ports"
+      target:
+        kind: Service