Refactor: cosign key types

[xopham]

Fixes no issue

Description

• several different types of keys (ecdsa, rsa) and references (email, kms) are being added to cosign and integrated for connaisseur
• the handling and identification of key type is being refactored to improve extension

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 #415 (ecbbcd8) into develop (f035e57) will increase coverage by 0.21%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #415      +/-   ##
===========================================
+ Coverage    94.24%   94.45%   +0.21%     
===========================================
  Files           22       22              
  Lines         1095     1101       +6     
===========================================
+ Hits          1032     1040       +8     
+ Misses          63       61       -2     

Impacted Files	Coverage Δ
connaisseur/validators/cosign/cosign_validator.py	98.83% <100.00%> (+2.58%)	⬆️

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[peterthomassen]

Nice! Nits only.

For the record, you may be able to do achieve a similar thing using new syntax from https://www.python.org/dev/peps/pep-0636/ (starting Python 3.10). However, it would be not what the new syntax is meant for, which reduces clarity, so I'd advise against it.

[peterthomassen]

lgtm!