By Luc Cohen NEW YORK (Reuters) -Former U.S. Treasury Secretary Steven Mnuchin took the stand on Thursday as a witness for the defense of Tom Barrack, a onetime fundraiser for former U.S. President Donald Trump who is on trial on charges of illegally acting as a foreign agent. Federal prosecutors in Brooklyn say Barrack, 75, used his influence with the Trump campaign and administration to push the United Arab Emirates’ interests without notifying the U.S. attorney general, as required by law. Barrack has pleaded not guilty. His lawyers have said his interactions with Middle Eastern officials w…