Cover signingKey round-trip in export tests

Extends the existing TestExport 'image update' case with a signingKey
block on the seeded ImageUpdateAutomation, asserting the new field
survives the kubeClient.Get + serialize path. Parallels how the
existing fixture exercises every other field on the resource.

Also patches the embedded CRD source under manifests/bases/image-
automation-controller/ to inject the signingKey.type schema property
into both v1 and v1beta2. The patch is transitional and should be
removed once the image-automation-controller release bundle includes
the new type field natively.

Signed-off-by: Hidde Beydals <hidde@hhh.computer>

manifests/bases/image-automation-controller/kustomization.yaml

@@ -13,3 +13,9 @@ patches:
     kind: Deployment
     name: image-automation-controller
   path: patch.yaml
+- target:
+    group: apiextensions.k8s.io
+    version: v1
+    kind: CustomResourceDefinition
+    name: imageupdateautomations.image.toolkit.fluxcd.io
+  path: crd-signing-key-type-patch.yaml