feat: support creating read-only OCI store from fs.FS

[Wwwsylvia]

Will support creating OCI store from tarball in next PR.

Resolves: #336
Signed-off-by: Lixia (Sylvia) Lei lixlei@microsoft.com

[codecov-commenter]

Codecov Report

Merging #367 (8c067fa) into main (253d5ec) will increase coverage by 0.20%.
The diff coverage is 77.00%.

@@            Coverage Diff             @@
##             main     #367      +/-   ##
==========================================
+ Coverage   71.84%   72.05%   +0.20%     
==========================================
  Files          43       45       +2     
  Lines        4103     4172      +69     
==========================================
+ Hits         2948     3006      +58     
- Misses        870      877       +7     
- Partials      285      289       +4     

Impacted Files	Coverage Δ
content/oci/oci.go	65.64% <ø> (ø)
registry/reference.go	68.42% <ø> (ø)
registry/remote/registry.go	67.41% <ø> (ø)
registry/remote/repository.go	68.22% <ø> (ø)
registry/repository.go	0.00% <ø> (ø)
content/oci/readonlyoci.go	66.12% <66.12%> (ø)
content/oci/readonlystorage.go	93.93% <93.93%> (ø)
content/oci/storage.go	63.76% <100.00%> (+1.66%)	⬆️
internal/syncutil/merge.go	71.42% <0.00%> (+1.78%)	⬆️

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[shizhMSFT]

LGTM