Fix: apply client.registerAPIRequestMiddleware to SDK auth requests

[EmilianoSanchez]

Usage example to add a custom header to SDK /auth requests:

  const ffClient = initialize(
    sdkKey, {
    identifier: id,
  });
  
  ffClient.registerAPIRequestMiddleware(([url, options]) => {
    if (url.includes('/client/auth')) {
      options.headers['X-Custom-Header'] = 'CustomValue';
    }
    return [url, options];
  });