Idea: Content addressing for buildpack registry

[abitrolly]

https://github.com/buildpacks/rfcs/blob/main/text/0022-client-side-buildpack-registry.md lacks the content addressing. If a buildpack is a reproducible binary blob, then any cloud object storage could be used to extract this blob by its hash. This removes the need to maintain a separate storage with addressing protocols, and the registry role can be reduced to just maintaining the meta info.