Make VerifyImageAttestation function public

[priyawadhwa]

We have a public VerifyImageSignature function, this is the analogous function for attestations.

[codecov]

Codecov Report

Merging #3156 (20c4a67) into main (af555d5) will decrease coverage by 0.03%.
Report is 8 commits behind head on main.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #3156      +/-   ##
==========================================
- Coverage   30.56%   30.54%   -0.03%     
==========================================
  Files         155      155              
  Lines        9801     9809       +8     
==========================================
  Hits         2996     2996              
- Misses       6359     6365       +6     
- Partials      446      448       +2     

Files Changed	Coverage Δ
pkg/cosign/verify.go	35.25% <0.00%> (-0.19%)	⬇️

... and 1 file with indirect coverage changes