Protobuf bundle support for subcommand clean

[steiza]

Summary

I think this really for-real completes #4470.

Testing is pretty easy! Run go run cmd/cosign/main.go clean <some image you signed before>.

Release Note

• Added protobuf bundle support to cosign clean

Documentation

N/A

[codecov]

Codecov Report

❌ Patch coverage is 0% with 45 lines in your changes missing coverage. Please review.
✅ Project coverage is 36.37%. Comparing base (2ef6022) to head (58944ca).
⚠️ Report is 595 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/cosign/cli/clean.go	0.00%	45 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4539      +/-   ##
==========================================
- Coverage   40.10%   36.37%   -3.73%     
==========================================
  Files         155      220      +65     
  Lines       10044    12271    +2227     
==========================================
+ Hits         4028     4464     +436     
- Misses       5530     7114    +1584     
- Partials      486      693     +207     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.