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>

cmd/flux/testdata/export/image-update.yaml

@@ -10,6 +10,10 @@ spec:
       author:
         email: fluxcdbot@users.noreply.github.com
         name: fluxcdbot
+      signingKey:
+        secretRef:
+          name: my-signing-key
+        type: ssh
   interval: 1m0s
   sourceRef:
     kind: GitRepository