Support more digest algorithms

[phbelitz]

Describe the feature
Connaisseur only supports sha256 as a digest algorithm. It's hard coded at many parts of the code. Multiple digests algorithms should be supported.

[peterthomassen]

Are such image references really used, or is this a hypothetical use case?

[reneeckstein]

@peterthomassen I just crossed one by trying to validate docker images from Jetstack (like cert-manager), they use SHA512
https://cert-manager.io/docs/installation/code-signing/#container-images--cosign

and I get the error from connaisseur
CONNAISSEUR rejected a request: The trust_root type <class 'connaisseur.trust_root.RSAKey'> is unsupported for a validator of typecosign. (not denied due to DETECTION_MODE)