An adviser to the United Arab Emirates with ties to current and former aides to President Trump is cooperating with the special counsel, Robert S. Mueller III, and gave testimony last week to a grand jury, according to two people familiar with the matter. Mr. Mueller appears to be examining the influence of foreign money on Mr. Trump’s political activities and has asked witnesses about the possibility that the adviser, George Nader, funneled money from the Emirates to the president’s political efforts.