feat: cosign kms support

[xopham]

fixes #325 #352

[codecov-commenter]

Codecov Report

Merging #360 (7cd5f2f) into develop (bd3a155) will decrease coverage by 0.16%.
The diff coverage is 81.81%.

@@             Coverage Diff             @@
##           develop     #360      +/-   ##
===========================================
- Coverage    97.02%   96.85%   -0.17%     
===========================================
  Files           21       21              
  Lines         1042     1050       +8     
===========================================
+ Hits          1011     1017       +6     
- Misses          31       33       +2     

Impacted Files	Coverage Δ
connaisseur/validators/cosign/cosign_validator.py	96.25% <81.81%> (-2.37%)	⬇️
...naisseur/validators/notaryv1/notaryv1_validator.py	95.79% <0.00%> (-0.04%)	⬇️
connaisseur/util.py	95.34% <0.00%> (ø)
connaisseur/config.py	97.32% <0.00%> (+0.02%)	⬆️

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