Feature: adds '--allow-insecure-registry' for cosign load

[mritunjaysharma394]

Summary

According to the issue #2986, we need in certain cases like that of "Tactical edge deployments for the Government", to 'load' an image to a registry that is had either NO tls cert, for the ability to tell cosign to ignore the cert, or use http instead.
This PR fixes #2986

Release Note

cosign load was added with --allow-insecure-registry to disable TLS verification when interacting with insecure (e.g. self-signed) container registries

Documentation

Yes, added it in the github docs

[znewman01]

Can we use the RegistryOpts directly? https://github.com/sigstore/cosign/blob/f21081a1820939cddc41c1c119632a6c489d57fd/cmd/cosign/cli/options/registry.go#LL41C1-L41C1

This means that you don't have to duplicate the code to create the Opts.

[codecov]

Codecov Report

Merging #3000 (2dd42b5) into main (f21081a) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #3000      +/-   ##
==========================================
- Coverage   30.25%   30.23%   -0.02%     
==========================================
  Files         151      151              
  Lines        9473     9478       +5     
==========================================
  Hits         2866     2866              
- Misses       6162     6167       +5     
  Partials      445      445              

Impacted Files	Coverage Δ
cmd/cosign/cli/load.go	0.00% <0.00%> (ø)
cmd/cosign/cli/options/load.go	0.00% <0.00%> (ø)

[mritunjaysharma394]

Thanks @znewman01 , just fixed that, I (wrongly) thought that we add opts for each command separately 😅 but yes we should try to be as DRY as we can be, thanks for the review! Let me know if it needs further improvements