Photo: Kyle Prudhomme/International Tennis Hall of Fame It’s fairly common to see obscenely rich people watching a match at the grass court at the International Tennis Hall of Fame in Newport, Rhode Island. Rarer, though, is the prospect of a billionaire on the court during a pro tournament — which is just what happened when billionaire Bill Ackman played in a doubles match at the Hall of Fame Open on Wednesday. The 59-year-old, known for his activist investing and frequent tweeting , teamed up