Add guard flag for experimental OCI 1.1 verify.

[vaikas]

Summary

Previously the experimental behaviour for verify defaulted to using experimental OCI 1.1 behvaiour. This seems to cause some problems with some registries. So default that behaviour to false, and you have to explicitly enable it with a flag.
Also, by adding it to checkopts, we can now flip this on/off for clients like policy-controller where the problems surfaced.

Release Note

Documentation

[codecov]

Codecov Report

Merging #3272 (1f9dd4c) into main (5ed63e0) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #3272      +/-   ##
==========================================
- Coverage   30.32%   30.31%   -0.02%     
==========================================
  Files         155      155              
  Lines        9853     9858       +5     
==========================================
  Hits         2988     2988              
- Misses       6418     6423       +5     
  Partials      447      447              

Files	Coverage Δ
cmd/cosign/cli/verify/verify.go	21.20% <0.00%> (-0.07%)	⬇️
cmd/cosign/cli/options/verify.go	0.00% <0.00%> (ø)
pkg/cosign/verify.go	34.57% <0.00%> (-0.04%)	⬇️

[cpanato]

Thanks

[hectorj2f]

LGTM