Protobuf bundle support for subcommands save and load

[steiza]

Summary

Continuing to make progress on #4470.

To test:

• Run local registries on port 1338 (default) and port 1337 (to test loading)

• Build and sign an image:

sigstore/sigstore-go/examples/sigstore-go-verification$ ko build .
sigstore/cosign$ go run cmd/cosign/main.go sign --key cosign.key --use-signing-config=false --tlog-upload=false localhost:1338/demo/sigstore-go-verification-ce4f5bf3233ac2b951c3667b2d19da3a:latest

• Save it from registry:

$ go run cmd/cosign/main.go save --dir save localhost:1338/demo/sigstore-go-verification-ce4f5bf3233ac2b951c3667b2d19da3a

• Load it into mirror registry:

$ go run cmd/cosign/main.go load --dir save localhost:1337/demo/sigstore-go-verification-ce4f5bf3233ac2b951c3667b2d19da3a

• Verify from mirror registry:

$ go run cmd/cosign/main.go verify --insecure-ignore-tlog=true --key cosign.pub localhost:1337/demo/sigstore-go-verification-ce4f5bf3233ac2b951c3667b2d19da3a

Release Note

• Added protobuf bundle support to cosign save and cosign load

Documentation

N/A

[codecov]

Codecov Report

❌ Patch coverage is 10.00000% with 72 lines in your changes missing coverage. Please review.
✅ Project coverage is 36.29%. Comparing base (2ef6022) to head (1759df5).
⚠️ Report is 598 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/oci/remote/write.go	12.28%	47 Missing and 3 partials ⚠️
cmd/cosign/cli/save.go	0.00%	19 Missing ⚠️
cmd/cosign/cli/options/verify.go	0.00%	2 Missing ⚠️
cmd/cosign/cli/load.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4538      +/-   ##
==========================================
- Coverage   40.10%   36.29%   -3.81%     
==========================================
  Files         155      220      +65     
  Lines       10044    12306    +2262     
==========================================
+ Hits         4028     4467     +439     
- Misses       5530     7145    +1615     
- Partials      486      694     +208     

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