Add CLI flag for OCIRepo verify secret

Signed-off-by: Matheus Pimenta <matheuscscp@gmail.com>

cmd/flux/testdata/oci/export_with_verify_secret.golden

@@ -0,0 +1,15 @@
+---
+apiVersion: source.toolkit.fluxcd.io/v1beta2
+kind: OCIRepository
+metadata:
+  name: podinfo
+  namespace: flux-system
+spec:
+  interval: 10m0s
+  ref:
+    tag: 6.3.5
+  url: oci://ghcr.io/stefanprodan/manifests/podinfo
+  verify:
+    provider: cosign
+    secretRef:
+      name: cosign-pub