Support artifactType, for images whose config.mediaType is not a config

[imjasonh]

If an index contains an image whose config.mediaType is not the normal config type (i.e., it's an artifact, packaged as an image for registry compatibility), the index's descriptor for the image will include artifactType.

This is work toward supporting the OCI 1.1 referrers fallback tag scheme, where referrers are managed by the client and pushed as indexes to the registry, tagged with :sha256-abcdef....

cc @jdolitsky

[codecov-commenter]

Codecov Report

Merging #1541 (95f906e) into main (d872232) will decrease coverage by 0.22%.
The diff coverage is 37.73%.

@@            Coverage Diff             @@
##             main    #1541      +/-   ##
==========================================
- Coverage   73.31%   73.09%   -0.22%     
==========================================
  Files         117      117              
  Lines        9015     9065      +50     
==========================================
+ Hits         6609     6626      +17     
- Misses       1746     1776      +30     
- Partials      660      663       +3     

Impacted Files	Coverage Δ
pkg/v1/manifest.go	100.00% <ø> (ø)
pkg/v1/mutate/mutate.go	72.81% <ø> (ø)
pkg/v1/remote/image.go	75.59% <0.00%> (-4.41%)	⬇️
pkg/v1/types/types.go	75.00% <0.00%> (-25.00%)	⬇️
pkg/v1/partial/with.go	60.17% <21.73%> (-4.36%)	⬇️
pkg/v1/remote/index.go	70.83% <57.14%> (-0.52%)	⬇️
pkg/v1/remote/descriptor.go	74.27% <100.00%> (+0.76%)	⬆️

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[jdolitsky]

👍

[jonjohnsonjr]

Overall LGTM. There are probably some things we missed, but let's let this bake for a bit before we cut a release.