The Birkin Bag is the ultimate status symbol : wildly expensive, ridiculously hard to get and immediately identifiable. Real Housewives collect it, hip-hop artists rap about it and “Sex and the City” once devoted an episode to it. Yet, according to Marisa Meltzer’s new biography, “ It Girl: The Life and Legacy of Jane Birkin ” (Atria Books, out Tuesday), the woman who inspired the pricey Hermés purse was the ultimate bohemian — and a scandalous one at that. She was condemned by the Vatican for