code: public CertifiedPublicKey convert(X509CertificateHolder cert)
      {
          if (cert == null) {
              return null;
          }
          return new BcX509CertifiedPublicKey(cert, this.factory);
      }
docs: Convert Bouncy Castle certificate holder.
  
  @param cert a certificate holder.
  @return a certified public key.
  @since 6.0M1