fix(digests): do not mandate sha256 as the only algorithm used for hashing blobs

[andaaron]

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

Attention: Patch coverage is 87.69231% with 16 lines in your changes are missing coverage. Please review.

Project coverage is 92.88%. Comparing base (be5ad66) to head (52dc22b).

Files	Patch %	Lines
pkg/storage/imagestore/imagestore.go	87.75%	4 Missing and 2 partials ⚠️
pkg/storage/gc/gc.go	20.00%	4 Missing ⚠️
pkg/storage/common/common.go	90.47%	2 Missing ⚠️
pkg/test/image-utils/images.go	92.00%	1 Missing and 1 partial ⚠️
pkg/test/image-utils/multiarch.go	86.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2075      +/-   ##
==========================================
+ Coverage   92.86%   92.88%   +0.02%     
==========================================
  Files         167      167              
  Lines       22077    22128      +51     
==========================================
+ Hits        20502    20554      +52     
+ Misses        982      980       -2     
- Partials      593      594       +1     

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[andaaron]

See also opencontainers/distribution-spec#494

[peusebiu]

lgtm

[ktarplee]

• @andaaron What else is needed on this PR? What you have looks good however I did not see the REST API changes. I expected there to be some changes necessary there.

[andaaron]

• @andaaron What else is needed on this PR? What you have looks good however I did not see the REST API changes. I expected there to be some changes necessary there.

We need the spec for specifying a non-canonical digest algorithm for the manifest in case of pushing by tag.
Once we have the spec we can implement that.

Otherwise this PR should be good for the blobs use case. But I don't know if there's any case in which the blobs use non-canonical digest algorithms while the manifest uses the canonical (sha256) digest algorithm.