By Richard Cowan WASHINGTON (Reuters) -A proposal to create a 9/11-style commission to investigate the Jan. 6 U.S. Capitol attack by a mob of Donald Trump’s supporters advanced in Congress on Tuesday even as one key Republican announced opposition and another suggested the former president could become a witness. The Democratic-backed proposal won approval in the House of Representatives Rules Committee, setting up a planned vote in the full House on Wednesday. The party-line vote in the committee, with Democrats in favor and Republicans against, came hours after Kevin McCarthy, the top House …