Cynthia Littleton Business EditorGary Ginsberg’s proximity to power during his years as a lawyer in the Clinton White House gave him a keen understanding of how influence works in the executive branch.But the spark for his new book, “First Friends: The Powerful, Unsung (and Unelected) People Who Shaped Our Presidents,” came from his observations of two other political figures: Gary Hart and Donald Trump.Ginsberg, the former head of corporate communications for News Corp.