feat: key interface

[phbelitz]

This commit adds an easy way to add additional key types into connaisseur. The first set of supported keys are: ECDSA, RSA[cosign], KMS[cosign] and Keyless[cosign] ande each gets automatically recognized.

Description

We eventually want to support more types of keys and even apply additional logic to specific types, while also have a common interface so that the types can be easily exchanged.

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)

[xopham]

some first minor comments

[codecov-commenter]

Codecov Report

Merging #540 (321fb01) into develop (21232a2) will increase coverage by 0.10%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #540      +/-   ##
===========================================
+ Coverage    96.69%   96.80%   +0.10%     
===========================================
  Files           22       22              
  Lines         1179     1219      +40     
===========================================
+ Hits          1140     1180      +40     
  Misses          39       39              

Impacted Files	Coverage Δ
connaisseur/exceptions.py	100.00% <100.00%> (ø)
connaisseur/trust_root.py	100.00% <100.00%> (ø)
connaisseur/validators/cosign/cosign_validator.py	99.24% <100.00%> (-0.04%)	⬇️
connaisseur/validators/notaryv1/key_store.py	100.00% <100.00%> (ø)
connaisseur/validators/notaryv1/notary.py	98.33% <100.00%> (ø)
...naisseur/validators/notaryv1/notaryv1_validator.py	98.31% <100.00%> (+0.01%)	⬆️
connaisseur/validators/notaryv1/trust_data.py	100.00% <100.00%> (ø)

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