Cover create image update signing flags

Adds golden-file tests for the new --signing-key-secret and
--signing-key-type flags: no-signing (baseline), default-gpg (asserts
type: gpg is rendered explicitly when only the secret is set), ssh,
and the two validation-error cases. Establishes
cmd/flux/testdata/create_image_update/ for future expansion of this
command's coverage.

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

cmd/flux/testdata/create_image_update/signing-default-gpg.yaml

@@ -0,0 +1,23 @@
+---
+apiVersion: image.toolkit.fluxcd.io/v1
+kind: ImageUpdateAutomation
+metadata:
+  name: flux-system
+  namespace: flux-system
+spec:
+  git:
+    checkout:
+      ref:
+        branch: main
+    commit:
+      author:
+        email: flux@example.com
+        name: flux
+      signingKey:
+        secretRef:
+          name: my-key
+        type: gpg
+  interval: 1m0s
+  sourceRef:
+    kind: GitRepository
+    name: flux-system