feat: cosign keyless support

[xopham]

Fixes #141

Description

• adds support for cosign keyless signatures via OIDC

:warning: WIP Notes:

• integration test should be added
• requires specification of rekor root cert and rekor url (host flag?)
• should allow to specify the issuer as well to avoid malicious OIDC from malicious identity provider

Checklist

• [ ] PR is rebased to/aimed at branch develop
• [ ] PR follows Contributing Guide
• [ ] Added tests (if necessary)
• [ ] Extended README/Documentation (if necessary)
• [ ] Adjusted versions of image and Helm chart in values.yaml and Chart.yaml (if necessary)

[codecov-commenter]

Codecov Report

Merging #407 (aaf2f1e) into develop (6102a6f) will decrease coverage by 0.14%. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #407      +/-   ##
===========================================
- Coverage    96.81%   96.66%   -0.15%     
===========================================
  Files           22       22              
  Lines         1226     1231       +5     
===========================================
+ Hits          1187     1190       +3     
- Misses          39       41       +2     

Impacted Files	Coverage Δ
connaisseur/validators/cosign/cosign_validator.py	97.91% <100.00%> (-1.37%)	:arrow_down:

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[phbelitz]

be aware of this: https://github.com/sse-secure-systems/connaisseur/pull/969#discussion_r1159669738