NEW YORK — A top executive at former President Donald Trump’s family business pleaded guilty Thursday to evading taxes on a free apartment and other perks, striking a deal with prosecutors that could make him a star witness against the company at a trial this fall.Allen Weisselberg, a senior Trump Organization adviser and formerly the company’s longtime chief financial officer, pleaded guilty to all 15 charges he faced in the case.In a low, somewhat hoarse voice, Weisselberg admitted taking in over $1.7 …