RITA Cosby, the pugnacious, former Fox New Channel reporter, joins the club tonight. Along with FNC's Greta Van Susteren and Headline News anchor Nancy Grace, Cosby will be initiated into a small, elite sorority of cable-news women who have - in just the last year or two - invented a new TV archtype. They are the hard-nosed hosts of prime-time news shows - a generation younger than Barbara Walters and Diane Sawyer, who have advanced to the top ranks through their charm as much as their intelligence. No one would ever call this new group of female news anchors "charming."