code: private static ClientAuthenticationMethod getPreferredAuthenticationMethod(OidcConfiguration config) {
          final ClientAuthenticationMethod configurationMethod = config.getClientAuthenticationMethod();
          if (configurationMethod == null) {
              return null;
          }
          
          if (!SUPPORTED_METHODS.contains(configurationMethod)) {
              throw new TechnicalException("Configured authentication method (" + configurationMethod + ") is not supported.");
          }
          
          return configurationMethod;
      }
docs: The preferred {@link ClientAuthenticationMethod} specified in the given
  {@link OidcConfiguration}, or <code>null</code> meaning that the a
  provider-supported method should be chosen.