Feat: Implement ES256K support for secp256k1 signed JWTs

[ThisIsMissEm]

This implements #391, and I've tested it for compatibility with the JWTs that the AT Protocol codebase produces (it's a little complex on the node.js/javascript side, so I haven't included that but I can if need be).

The error I mentioned in that ticket was because I was accidentally doing Validation::new(Algorithm::ES256), instead of &Validation::new(Algorithm::ES256K), when decoding the JWT, which, super silly mistake, but I caught it today!

This would unblock: https://github.com/mike-engel/jwt-cli/issues/402