feat/trigger: enable trigger to send to different namespace (#2016)

After this change, when the namepsace is specified in the subscriber
the URI can be resolved correctly to the specified namespace instead
of forcing it to the be same as the trigger namespace.

pkg/reconciler/trigger/trigger.go

@@ -180,7 +180,7 @@ func (r *Reconciler) FinalizeKind(ctx context.Context, t *brokerv1beta1.Trigger)
 }
 
 func (r *Reconciler) resolveSubscriber(ctx context.Context, t *brokerv1beta1.Trigger, b *brokerv1beta1.Broker) error {
-	if t.Spec.Subscriber.Ref != nil {
+	if t.Spec.Subscriber.Ref != nil && t.Spec.Subscriber.Ref.Namespace == "" {
 		// To call URIFromDestination(dest apisv1alpha1.Destination, parent interface{}), dest.Ref must have a Namespace
 		// We will use the Namespace of Trigger as the Namespace of dest.Ref
 		t.Spec.Subscriber.Ref.Namespace = t.GetNamespace()