fix: remove subject existence check, add media type check

[wangxiaoxuan273]

Resolves #42
Resolves #50
Resolves #49

PR description:

1. This pr removes subject existence check, as according to the distribution spec, subject existence check is not needed.
2. This pr removes subject from the References() function. For subject we don't need to check existence, and we need to verify its media type. This is quite different from the validation of layers, blobs and configs (all included in References()), and therefore it's better to move subject out of References(), as all elements of References() are checked in a loop and it's very hard to tell which one is the subject and differentiate its check.

Signed-off-by: wangxiaoxuan273 [email protected]

[codecov-commenter]

Codecov Report

Merging #52 (7feecaf) into main (884c54b) will decrease coverage by 0.01%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##             main      #52      +/-   ##
==========================================
- Coverage   56.31%   56.29%   -0.02%     
==========================================
  Files         108      108              
  Lines       11211    11210       -1     
==========================================
- Hits         6313     6311       -2     
- Misses       4188     4191       +3     
+ Partials      710      708       -2     

Impacted Files	Coverage Δ
manifest/ociartifact/manifest.go	53.19% <ø> (-2.81%)	⬇️
manifest/ocischema/manifest.go	74.19% <ø> (+3.42%)	⬆️
registry/storage/ocimanifesthandler.go	58.18% <0.00%> (-5.82%)	⬇️
registry/storage/ociartifactmanifesthandler.go	63.75% <100.00%> (+4.92%)	⬆️

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[shizhMSFT]

LGTM