WASHINGTON, July 22— Throughout the debate over national health insurance, the hottest issue has been how to pay for it. Today, that question boiled down to the cost of pizza. At a Senate hearing today, Allan S. Huston, president of Pizza Hut, a division of Pepsico Inc., said that requiring employers to pay for their workers' insurance would mean lost jobs, lower wages and closed businesses. These are the arguments that small businesses and operations like Pizza Hut, with its many low-wage...