Return ReferencePolicy CRD, deprecated, for v0.5.0

[youngnick]

To make migration easier, we should deprecate the v1alpha2 ReferencePolicy resource in v0.5.0, rather than removing it.

This means we'll need to add the types and CRD back, and figure out how to get the deprecation field and warning set on the generated CRD object. (This will need to use https://kubernetes.io/docs/tasks/extend-kubernetes/custom-resources/custom-resource-definition-versioning/#version-deprecation).

ReferencePolicy will then be removed in v0.6.0.

[robscott]

/assign