Support for OCI 1.1+ referrers via fallback tag

[jdolitsky]

Builds on top of #1541

Crane subcommands "attach" and "refs" dropped in this PR, but can be found here: https://github.com/google/go-containerregistry/compare/main...jdolitsky:attach-pt-3?expand=1

cc @imjasonh

[imjasonh]

This looks good!

[codecov-commenter]

Codecov Report

Merging #1543 (1e16e45) into main (da1008f) will decrease coverage by 0.31%.
The diff coverage is 52.70%.

@@            Coverage Diff             @@
##             main    #1543      +/-   ##
==========================================
- Coverage   73.07%   72.77%   -0.31%     
==========================================
  Files         117      118       +1     
  Lines        9082     9230     +148     
==========================================
+ Hits         6637     6717      +80     
- Misses       1780     1829      +49     
- Partials      665      684      +19     

Impacted Files	Coverage Δ
pkg/v1/manifest.go	100.00% <ø> (ø)
pkg/v1/remote/delete.go	58.62% <ø> (ø)
pkg/v1/mutate/mutate.go	69.01% <10.52%> (-3.80%)	⬇️
pkg/v1/remote/referrers.go	40.00% <40.00%> (ø)
pkg/v1/remote/write.go	63.12% <55.95%> (-0.99%)	⬇️
pkg/v1/remote/descriptor.go	73.84% <61.53%> (-0.44%)	⬇️
pkg/v1/mutate/image.go	69.56% <100.00%> (+0.33%)	⬆️
pkg/v1/mutate/index.go	79.33% <100.00%> (+0.34%)	⬆️
pkg/v1/remote/options.go	77.77% <100.00%> (+1.41%)	⬆️
... and 2 more

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[imjasonh]

@jonjohnsonjr anything else?