President-elect Donald J. Trump is turning to two of his most prominent billionaire supporters, Elon Musk and Vivek Ramaswamy, to overhaul the federal government, saying on Tuesday that they would lead what he called the Department of Government Efficiency. Calling the department “the Manhattan Project” of this era, Mr. Trump said it would propel “drastic change” throughout the federal bureaucracy by July 4, 2026.