Approach to artifact manifest seems to be out of date

[waynr]

In #115 a draft version of artifact support was repeatedly mentioned. If you click to that link, the file no longer exists.

There is a very long and contentious discussion about this in the image-spec repo. I didn't read the entire discussion, but there is a good summary as to what motivated the change. They also provide guidance on using the long-standing image manifest type as an artifact manifest type, essentially codifying a convention established by the community of registries and clients over the years:

• Define image manifest artifactType and guidance opencontainers/image-spec#1043
• image manifest: support use as artifact opencontainers/image-spec#1030

To this end, I think it makes sense for this repo to at least support using an Image Manifest as an Artifact Manifest, which is codified in the current v1.1.0-rc5 of OCI Image Spec. That said, I don't necessarily think this repo needs to remove Artifact Manifest just yet (maybe wait until v1.1.0 is officially released).

[waynr]

This was fixed in #145