By Richard Cowan WASHINGTON (Reuters) -In a potential breakthrough toward the first significant new U.S. gun law in decades, a bipartisan group of senators on Sunday announced an agreement on a framework for a firearms safety bill with enough Republican support to advance in the narrowly divided Senate. The plan, lauded by President Joe Biden, includes support for state “red flag” laws keeping firearms from potentially dangerous people, tougher criminal background checks for gun buyers under age 21 and a crackdown on “straw purchases” by people buying weapons for others who could not pass a ba…