* Add the initial caching type definitions.
This is largely carried over from https://github.com/knative/serving/pull/1972
* Incorporate feedback from @evankanderson
* Add the CRD for the type definition.
This adds all of the elements needed to bootstrap the knative/caching repo.
I've tried to limit this to largely boilerplate elements so that it can more easily be recycled as we create other repos.