code: public Future<OAuth2AccessToken> getAccessTokenClientCredentialsGrant(
              OAuthAsyncRequestCallback<OAuth2AccessToken> callback) {
          final OAuthRequest request = createAccessTokenClientCredentialsGrantRequest(null);
  
          return sendAccessTokenRequestAsync(request, callback);
      }
docs: Start the request to retrieve the access token using client-credentials grant. The optionally provided callback
  will be called with the Token when it is available.
  
  @param callback optional callback
  @return Future