CAS registry and template implementation in github.com/wking/casengine

[wking]

Very early work in https://github.com/wking/casengine

Once this settles down, it probably merits a link from this repo. I've kept it separate because I want to see the CAS-engine registry landed in image-spec and image-tools (or a separate CAS-only project) under the OCI. It is very orthogonal to the rest ref-engines and ref-engine discovery stuff in this repository.