By Alexander Villegas VALPARAISO, Chile (Reuters) -Chilean leftist Gabriel Boric was sworn in as president on Friday, vowing to listen to all sides while warning of the challenges ahead, as the Andean country marked the sharpest shift in its politics since the return to democracy three decades ago after the bloody dictatorship of General Augusto Pinochet. At the Congress building in the port city of Valparaiso, Boric, a 36-year-old, tattooed former protest leader and lawmaker, took the presidential sash from outgoing billionaire President Sebastian Pinera, making him the country’s youngest-eve…