feat: add oci image index support

[piksel]

This PR simply appends application/vnd.oci.image.index.v1+json to the Accepted header when querying for image manifests. This format was added to docker buildx v0.10 and for images that use it, causes watchtower 1.5.1 to fail checking for changes using HEAD requests (since the repository returns a 404 when the Accepted mime type is not present for the manifest).

When testing locally, watchtower was able to get the digests for all the found problematic images.

Fixes #1528.

[codecov]

Codecov Report

Base: 66.19% // Head: 66.20% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (42be14b) compared to base (c16ac96).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1533      +/-   ##
==========================================
+ Coverage   66.19%   66.20%   +0.01%     
==========================================
  Files          24       24              
  Lines        2313     2314       +1     
==========================================
+ Hits         1531     1532       +1     
  Misses        682      682              
  Partials      100      100              

Impacted Files	Coverage Δ
pkg/registry/digest/digest.go	43.52% <100.00%> (+0.67%)	⬆️

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