Support OCI layout tarballs in crane load

[Haihan-Jiang]

Fixes #2109.

Summary

• let crane.Load fall back from Docker tarball parsing to single-image OCI layout tarballs
• safely extract OCI layout tarballs to a temporary directory and materialize the image before cleanup
• update crane push help text now that non-directory inputs can be Docker or OCI layout tarballs
• add coverage for loading an OCI layout tarball created from crane.SaveOCI

Testing

• go test ./pkg/crane -run TestCraneLoadOCILayoutTarball -count=1
• go test ./pkg/crane ./cmd/crane/cmd -count=1
• go test ./...

[Haihan-Jiang]

This is ready for review. The change lets crane load accept OCI layout tarballs through the existing push path, with coverage in pkg/crane. The visible checks are green.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 51.47059% with 33 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.87%. Comparing base (31df54c) to head (61d66a3).
⚠️ Report is 19 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/crane/push.go	50.74%	19 Missing and 14 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2320      +/-   ##
==========================================
- Coverage   56.91%   56.87%   -0.04%     
==========================================
  Files         165      165              
  Lines       11362    11428      +66     
==========================================
+ Hits         6467     6500      +33     
- Misses       4134     4153      +19     
- Partials      761      775      +14     

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