Add valid signatures for jwt_vc_json section

[javereec]

Addressing #233

I've changed the jwt_vc_json example and added the necessary information to allow verification of the signatures.

The key proof jwt

{
  "typ": "openid4vci-proof+jwt",
  "alg": "ES256",
  "kid": "did:jwk:eyJraWQiOiJ1cm46aWV0ZjpwYXJhbXM6b2F1dGg6andrLXRodW1icHJpbnQ6c2hhLTI1NjpWYkpPU3ZqeFU2TDhDN0dVTzRkc2hJWVYzemJ2RndrWUI0M1lKNUt0dDhFIiwia3R5IjoiRUMiLCJjcnYiOiJQLTI1NiIsImFsZyI6IkVTMjU2IiwieCI6Ik1kQy1PS3E0QVFKZlZDWDV6cFFvTDhqNFZFZnZQWDk4dFU5aHhjTlhHcm8iLCJ5IjoibnNXbmZiNk5Xc0szOUJILWhBYVNrQ1NlNEJ5bWVOc2NKRV9zYUQzRDNiTSJ9"
}.
{
  "aud": "https://credential-issuer.example.com",
  "iat": 1728071462,
  "nonce": "tZignsnFbp"
}

The credential in the response

{
  "typ": "JWT",
  "alg": "ES256",
  "kid": "urn:ietf:params:oauth:jwk-thumbprint:sha-256:mlUpog7vEewFBem6Ul09c2dtTwc8dFzVpIDX3sqGWW0"
}
{
  "vc": {
    "@context": [
      "https://www.w3.org/2018/credentials/v1",
      "https://www.w3.org/2018/credentials/examples/v1"
    ],
    "id": "https://credential-issuer.example.com/credentials/3732",
    "type": [
      "VerifiableCredential",
      "UniversityDegreeCredential"
    ],
    "issuer": "https://credential-issuer.example.com",
    "issuanceDate": "2025-01-01T00:00:00Z",
    "credentialSubject": {
      "id": "did:jwk:eyJraWQiOiJ1cm46aWV0ZjpwYXJhbXM6b2F1dGg6andrLXRodW1icHJpbnQ6c2hhLTI1NjpWYkpPU3ZqeFU2TDhDN0dVTzRkc2hJWVYzemJ2RndrWUI0M1lKNUt0dDhFIiwia3R5IjoiRUMiLCJjcnYiOiJQLTI1NiIsImFsZyI6IkVTMjU2IiwieCI6Ik1kQy1PS3E0QVFKZlZDWDV6cFFvTDhqNFZFZnZQWDk4dFU5aHhjTlhHcm8iLCJ5IjoibnNXbmZiNk5Xc0szOUJILWhBYVNrQ1NlNEJ5bWVOc2NKRV9zYUQzRDNiTSJ9",
      "degree": {
        "type": "BachelorDegree",
        "name": "Bachelor of Science and Arts"
      }
    }
  },
  "iss": "https://credential-issuer.example.com",
  "nbf": 1735689600,
  "jti": "https://credential-issuer.example.com/credentials/3732",
  "sub": "did:jwk:eyJraWQiOiJ1cm46aWV0ZjpwYXJhbXM6b2F1dGg6andrLXRodW1icHJpbnQ6c2hhLTI1NjpWYkpPU3ZqeFU2TDhDN0dVTzRkc2hJWVYzemJ2RndrWUI0M1lKNUt0dDhFIiwia3R5IjoiRUMiLCJjcnYiOiJQLTI1NiIsImFsZyI6IkVTMjU2IiwieCI6Ik1kQy1PS3E0QVFKZlZDWDV6cFFvTDhqNFZFZnZQWDk4dFU5aHhjTlhHcm8iLCJ5IjoibnNXbmZiNk5Xc0szOUJILWhBYVNrQ1NlNEJ5bWVOc2NKRV9zYUQzRDNiTSJ9"
}