By Luc Cohen NEW YORK (Reuters) -Former President Donald Trump may be called as a witness at the criminal trial of Thomas Barrack, a private equity investor and former Trump fundraiser charged with acting as an unregistered agent for the United Arab Emirates, a U.S. judge said on Monday. U.S. District Judge Brian Cogan in Brooklyn told several prospective jurors for Barrack’s trial about the prospect of Trump testifying to gauge whether they might be biased against the defendant. Prosecutors said Barrack, a former chairman of the company now called DigitalBridge Group Inc, tried to exploit his…