feat: Support cosign --k8s-keychain flag

[marckn0x]

Fixes #390

Description

Add configuration option k8s_keychain for cosign validator that will pass the flag --k8s-keychain to cosign when set to true. This allows cosign to pick up registry credentials from the environment (see sigstore/cosign#972 for details)

Checklist

• PR is rebased to/aimed at branch develop
• PR follows Contributing Guide
• Added tests (if necessary)
• Extended README/Documentation (if necessary)

[marckn0x]

I can add a test if necessary but it's a small change and any test would probably be contrived. Let me know if you think I should.

This is ready for review.

[marckn0x]

@phbelitz Doesn't look like I have permission to request a review. Would you mind taking a look at some point?

[codecov-commenter]

Codecov Report

Merging #551 (ec8991e) into develop (09ddf75) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #551      +/-   ##
===========================================
+ Coverage    94.23%   94.24%   +0.01%     
===========================================
  Files           22       22              
  Lines         1127     1129       +2     
===========================================
+ Hits          1062     1064       +2     
  Misses          65       65              

Impacted Files	Coverage Δ
connaisseur/validators/cosign/cosign_validator.py	97.80% <100.00%> (+0.04%)	⬆️

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[xopham]

@marckn0x this is an awesome feature 🎉 Must have slipped my attention, but great that you picked it up 🚀

I have some comments. Did you make a functional test of the feature in your environment?
I'd prefer if a small test was added to the pytests, maybe around here, though I admit this may be of limited use.

Please, adjust the commit message to something like feat: Support cosign --k8s-keychain flag in order to follow semantic and conventional commit messages as described in the contributing guide (just noticed the link in the PR template is broken 🙈).

[marckn0x]

I added a test in the location suggested; thanks!

[xopham]

Requested a few last changes and after that we're good to go 🚀

[xopham]

I made a proposal for the tests. If that works for you, feel free to squash and I'd be happy to approve 🙂

[xopham]

@marckn0x thanks for the great contribution 🚀