Add Docker certs.d support to crane

[Haihan-Jiang]

Summary

• add a crane transport wrapper that loads Docker-style per-registry certificate directories
• support registry CA files (*.crt) and client certificate/key pairs (*.cert/*.key)
• enable the wrapper for crane while keeping the existing transport headers, warnings, and insecure handling intact

Details

Docker supports custom registry certificates under certs.d/<registry-host>/. This teaches crane to look up the request registry host, clone the base transport for that host, and apply matching certs only to that registry. Missing cert directories are ignored, while incomplete client certificate pairs return an early configuration error.

Updates #211.

Testing

• go test ./cmd/crane/cmd
• go test ./cmd/crane/... ./pkg/crane/...
• go test ./pkg/v1/remote/transport
• PATH="$PATH:$(go env GOPATH)/bin" ./hack/presubmit.sh

[Haihan-Jiang]

This is still ready for review from my side. It adds Docker certs.d support to crane while keeping the existing keychain path intact; could someone take a look when convenient?

[codecov-commenter]

Codecov Report

❌ Patch coverage is 53.33333% with 42 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.89%. Comparing base (31df54c) to head (83fd770).
⚠️ Report is 19 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/crane/cmd/docker_certs.go	53.93%	25 Missing and 16 partials ⚠️
cmd/crane/cmd/root.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2319      +/-   ##
==========================================
- Coverage   56.91%   56.89%   -0.03%     
==========================================
  Files         165      166       +1     
  Lines       11362    11451      +89     
==========================================
+ Hits         6467     6515      +48     
- Misses       4134     4159      +25     
- Partials      761      777      +16     

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[Subserial]

Hi, just leaving a comment to say that I am still reviewing all of your PRs, it's just taking a bit of time to process new feature work. A bit late but I've approved running tests on each one.