feat: support cosign rsa keys

[xopham]

Fixes #201

Description

• support of rsa public keys for cosign validator is added

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 #416 (7a6c16e) into develop (b7b2f05) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #416      +/-   ##
===========================================
+ Coverage    94.45%   94.52%   +0.06%     
===========================================
  Files           22       22              
  Lines         1101     1114      +13     
===========================================
+ Hits          1040     1053      +13     
  Misses          61       61              

Impacted Files	Coverage Δ
connaisseur/crypto.py	100.00% <100.00%> (ø)
connaisseur/validators/cosign/cosign_validator.py	98.91% <100.00%> (+0.07%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b7b2f05...7a6c16e. Read the comment docs.

[timothy-spencer]

Any motion on this? I'm trying to use the pubkey half of an AWS kms asymmetric key, and it keeps saying The trust_root type <class 'connaisseur.trust_root.RSAKey'> is unsupported for a validator of typecosign. I can verify using cosign and this pubkey by hand, but connaisseur is saying NOPE. Seems like this would fix that.

[phbelitz]

@timothy-spencer i will have a look next week 🥸