Convert Proof class into JwtProof, CwtProof, and LdpVpProof as they are mutually exclusive

### Description

Following the review comment: https://github.com/keycloak/keycloak/pull/30692#discussion_r1667625499 and https://github.com/keycloak/keycloak/pull/30692#discussion_r1667626420

"jwt", "cwt" and "ldp_vp" are mutually exclusive.

IMO, it might be better to create JwtProof, CwtProof, and LdpVpProof class for each proof types.