Detect if user supplied a valid protobuf bundle

[steiza]

Summary

Fixes #3930

Several users, when trying out the new verification with protobuf bundles, first left off --new-bundle-format. We can detect if the user supplies a protobuf bundle, and assume they want to use the new verification flow.

Release Note

• Detect if you used a new protobuf bundle for cosign verify-blob and cosign verify-blob-attestation, even if you leave off --new-bundle-format

Documentation

N/A

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 36.36%. Comparing base (2ef6022) to head (2f7d720).
Report is 238 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3931      +/-   ##
==========================================
- Coverage   40.10%   36.36%   -3.74%     
==========================================
  Files         155      209      +54     
  Lines       10044    13382    +3338     
==========================================
+ Hits         4028     4867     +839     
- Misses       5530     7889    +2359     
- Partials      486      626     +140     

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[haydentherapper]

Nice!