Looking, as did the Roman deity Janus, forward and back, San Francisco Opera has announced its upcoming 100th season, one that is both long on ambition and heavy on nostalgia. In a livestreamed news conference early Wednesday evening, general director Matthew Shilvock and music director Eun Sun Kim unveiled big plans for the company’s 2022-2023 centennial, beginning with an opening night concert and an Opera Ball in the City Hall rotunda on Sept. 9 and encompassing eight subsequent mainstage operas and several additional concerts and celebratory events through June 30, 2023.