Extend subscriber for subscription/trigger to other resources apart from KSVC

spec:
 channel:
  apiVersion: messaging.knative.dev/v1
  kind: InMemoryChannel
  name: in-memory-channel
 subscriber:
  ref:
   apiVersion: serving.knative.dev/v1
   kind: Service
   name: hello-openshift

spec:
 channel:
  apiVersion: messaging.knative.dev/v1
  kind: InMemoryChannel
  name: default
 delivery:
  retry: 5
  backoffPolicy: exponential # or linear
  backoffDelay: "PT0.5S" # or ISO8601 duration
  deadLetterSink:
  ref:
   apiVersion: serving.knative.dev/v1
   kind: Service
   name: error-handler
 subscriber:
  uri: http://doesnotexist.default.svc.cluster.local