Caroline Framke Chief TV CriticChristine Baranski’s career is so vast and varied — from starring roles in “The Good Wife” and its spinoff “The Good Fight,” to spending decades in New York City theater, to stealing scenes as the “Mamma Mia” franchise’s resident siren — that it’s genuinely shocking when she points out that she’s never been in an onscreen period piece “with wigs, corsets, language, the whole thing.”HBO’s “The Gilded Age,” a glamorous new post-Civil War era drama from “Downton Abbey” creator Julian Fellowes, embodies that “whole thing” and then some. So when Fellowes reached out to offer Baranski a part that occupies what many “Downton” fans will recognize as the Dowager Countess role (i.e.