SD-JWT Issuer: Add Holder Public Key Claim (JWK)

[sandrask]

If the Issuer wants to enable Holder Binding, it MAY include a public key associated with the Holder, or a reference thereto.

We will use confirmation "cnf" claim as defined in [RFC7800] to include raw public key by value in SD-JWT.

In this first iteration of "cnf" claim we will use jwk confirmation method and include public key information in JWK format. Other confirmation methods such as "jwe", "kid" and "jku" may be implemented at later time (if required).