Allow ImageUpdateAutomations in object refs

This adds ImageUpdateAutomation as a kind allowed by cross-namespace
object references here. That has two effects:

 - ImageUpdateAutomation objects can be the source of events; and,
 - ImageUpdateAutomation objects can be the target of webhook
   triggers.

Of these, the first is certainly desirable (e.g,. now you can post a
Slack message when automation fails). The second may be useful, though
automations will more usually be triggered by ImageRepository objects
changing, and anyway doesn't hurt.

Signed-off-by: Michael Bridgen <michael@weave.works>

api/v1beta1/reference_types.go

@@ -24,7 +24,7 @@ type CrossNamespaceObjectReference struct {
 	APIVersion string `json:"apiVersion,omitempty"`
 
 	// Kind of the referent
-	// +kubebuilder:validation:Enum=Bucket;GitRepository;Kustomization;HelmRelease;HelmChart;HelmRepository;ImageRepository
+	// +kubebuilder:validation:Enum=Bucket;GitRepository;Kustomization;HelmRelease;HelmChart;HelmRepository;ImageRepository;ImageUpdateAutomation
 	// +required
 	Kind string `json:"kind,omitempty"`