Stop using v1.ObjectReference in kpack types

[chenbh]

example

We should stop using v1.ObjectReference embeds in our types as they contain way more fields than needed.

For example, we can fill in namespace, resourceVersion, and uid and they'll actually get persisted even though we don't use them at all

apiVersion: kpack.io/v1alpha2
kind: Builder
metadata:
  name: my-builder
spec:
  stack:
    name: bionic-stack
    kind: ClusterStack
    namespace: default
    resourceVersion: "74709881"
    uid: 811bec3c-f527-4e12-b3bd-d7837c331d81
  ...