Additional JWK members.
Key type (e.g. "OKP", "EC", "RSA").
OptionalcrvCryptographic curve name (e.g. "Ed25519", "X25519", "P-256").
OptionalalgAlgorithm intended for use with the key (e.g. "EdDSA").
OptionaluseKey usage: "sig" for signing, "enc" for encryption.
OptionalkidKey ID.
OptionalxBase64url-encoded public key coordinate.
OptionaldBase64url-encoded private key scalar.
JSON Web Key (JWK) representation per RFC 7517.