code: public static AsymmetricKeyCredential create(final String clientId,
              final PrivateKey key, final X509Certificate publicCertificate) {
          return new AsymmetricKeyCredential(clientId, key, publicCertificate);
      }
docs: Static method to create KeyCredential instance.
  
  @param clientId
  Identifier of the client requesting the token.
  @param key
  RSA private key to sign the assertion.
  @param publicCertificate
  Public certificate used for thumb print.
  @return KeyCredential instance