Expose credential public key as a PublicKey object

[iaik-jheher]

As discussed in #288; this allows consumers to handle the credential's public key without needing specific CBOR/COSE logic.

I considered @Getter(lazy=true)-based caching, but ended up deciding against it for three reasons:

• Decoding will likely not be performed by many consumers
• Decoding is a very fast operation
• Decoding may throw

[emlun]

Thanks, I finally have time to properly dig into these. I'll make some small modifications but this looks good to me!

Would you like to be credited in the release notes? If yes, would you prefer by real name or by GitHub username?

[iaik-jheher]

Sure, I'd appreciate that if you think it appropriate. I'll have to check with my employer about details; I'll get back to you on that.

[iaik-jheher]

We'd prefer attribution to take the form Jakob Heher (A-SIT) if possible. Thanks for offering!