code: public ServiceFuture<Void> beginDownloadAsync(String resourceGroupName, String virtualWANName, GetVpnSitesConfigurationRequest request, final ServiceCallback<Void> serviceCallback) {
          return ServiceFuture.fromResponse(beginDownloadWithServiceResponseAsync(resourceGroupName, virtualWANName, request), serviceCallback);
      }
docs: Gives the sas-url to download the configurations for vpn-sites in a resource group.
  
  @param resourceGroupName The resource group name.
  @param virtualWANName The name of the VirtualWAN for which configuration of all vpn-sites is needed.
  @param request Parameters supplied to download vpn-sites configuration.
  @param serviceCallback the async ServiceCallback to handle successful and failed responses.
  @throws IllegalArgumentException thrown if parameters fail the validation
  @return the {@link ServiceFuture} object