support --platform for verify-attestation

[imjasonh]

Summary

Support cosign verify-attestation <img> --platform=linux/arm64 ...

This flag was added to cosign download attestation in #2980, but this promotes the flag to a general-purpose registry option, which means it can be reused by cosign download <sbom|attestation> and verify-attestation, and others.

Release Note

• Added broader support for --platform

Documentation

Generated docs are probably sufficient.

[codecov]

Codecov Report

Merging #3181 (9c49b28) into main (1c3208b) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #3181      +/-   ##
==========================================
- Coverage   30.54%   30.53%   -0.01%     
==========================================
  Files         155      155              
  Lines        9809     9811       +2     
==========================================
  Hits         2996     2996              
- Misses       6365     6367       +2     
  Partials      448      448              

Files Changed	Coverage Δ
cmd/cosign/cli/download.go	0.00% <0.00%> (ø)
cmd/cosign/cli/options/download.go	0.00% <ø> (ø)
cmd/cosign/cli/options/registry.go	0.00% <0.00%> (ø)
cmd/cosign/cli/verify/verify_attestation.go	3.33% <ø> (ø)

[cpanato]

@imjasonh can you please update the docs? :)

[github-actions]

This PR is stale because it has been open 30 days with no activity. Remove stale label or comment or this will be closed in 10 days.

[github-actions]

This PR was closed because it has been stalled for 10 days with no activity.