CON Edison, the utility that powers the Rockefeller Center Christmas tree, has installed a Festivus pole in one of its Bronx offices. The unadorned aluminum poles are the brainchild of the Frank Costanza "Seinfeld" character who, in a 1997 episode of the classic sitcom, invented Festivus in reaction to the growing commercialization of Christmas."We had no Christmas decorations here this year, so I got the pole instead," a Con Ed source told The Post's Philip Recchia. The Wagner Company of Milwaukee, which sells the poles for $38 a pop, confirmed Con Ed's Grinch-like purchase. Wagner also says it sold a pole to Wall Street powerhouse Merrill Lynch.