CredentialRepositoryV2: more ways to create COSE keys would be convenient

[iaik-jheher]

Picking this back up since our prototype from last year is finally being moved towards production :)

CredentialRecord (the interface type for CredentialRepositoryV2) requires implementers to provide a getter the COSE-encoded public key. Its javadoc refers to a convenience helper cosePublicKeyFromEs256Raw for converting from ES256 raw to COSE.

Would it be possible to add an additional helper for converting from java PublicKey to COSE? (This is adjacent to #288 from last year.)

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Our business case: we need to submit the credential public key to an external server on registration, and receive a certificate for that public key in return. We store the certificate indexed by the credential id, which allows us to find it again at authentication time.

At this point, we end up with a X509Certificate, which provides a java.security.PublicKey. Similar to #288, I am trying to go from there to the COSE_Key required by the library without implementing COSE encoding.