LONDON, Jan 15 (Reuters) - A major erosion of the Federal Reserve's independence would be ​negative for the U.S. credit rating, ‌Fitch's top sovereign analyst said on Thursday, with any ‌sign that the dollar's top global currency crown could slip the most critical issue. The independence of the Fed has been thrown squarely into ⁠the spotlight this ‌week after U.S. prosecutors launched an investigation into the central bank's head ‍Jerome Powell over cost overruns to refurbishment work at its headquarters. ...