Add --offline flag to force offline verification

[priyawadhwa]

Summary

Release Note

Documentation

[codecov-commenter]

Codecov Report

Merging #2427 (e7f3160) into main (40fa54c) will increase coverage by 0.31%.
The diff coverage is 22.77%.

❗ Current head e7f3160 differs from pull request most recent head 7757789. Consider uploading reports for the commit 7757789 to get more accurate results

@@            Coverage Diff             @@
##             main    #2427      +/-   ##
==========================================
+ Coverage   30.12%   30.44%   +0.31%     
==========================================
  Files         136      136              
  Lines        8459     8400      -59     
==========================================
+ Hits         2548     2557       +9     
+ Misses       5582     5508      -74     
- Partials      329      335       +6     

Impacted Files	Coverage Δ
cmd/cosign/cli/dockerfile.go	0.00% <0.00%> (ø)
cmd/cosign/cli/manifest.go	0.00% <0.00%> (ø)
cmd/cosign/cli/options/certificate.go	0.00% <0.00%> (ø)
cmd/cosign/cli/options/verify.go	0.00% <0.00%> (ø)
cmd/cosign/cli/verify.go	0.00% <0.00%> (ø)
cmd/cosign/cli/verify/verify.go	18.46% <0.00%> (-0.47%)	⬇️
cmd/cosign/cli/verify/verify_attestation.go	0.00% <0.00%> (ø)
cmd/cosign/cli/verify/verify_blob.go	52.34% <25.00%> (+5.93%)	⬆️
pkg/cosign/verify.go	36.64% <42.85%> (+2.17%)	⬆️
... and 2 more

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[priyawadhwa]

@asraa ptal whenever you can! :)

[znewman01]

This LGTM but will let someone else pull the trigger on approval