By CAMPBELL ROBERTSON, ALLISON HOPE WEINER and WILLIAM K. RASHBAUM The New York Post is cooperating with a federal investigation into whether a longtime contributor for the Page Six gossip column — the avidly read daily log of wrongdoing, double-dealing and sexual indiscretions by celebrities both minor and major — tried to extort money from a California billionaire, according to a spokesman for the newspaper. Several people involved in the investigation said the reporter, Jared Paul Stern, had been captured on a video recording demanding a $100,000 payment and a monthly stipend of $10,000 from Ronald W. Burkle in return...