Gene Maddaus Senior Media WriterStars of the “Avengers” franchise — Mark Ruffalo, Zoe Saldana, Scarlett Johansson, Paul Rudd, Chris Evans, Don Cheadle and Robert Downey, Jr. — joined together for a virtual fundraiser on Tuesday for the Joe Biden campaign.Kamala Harris also appeared for the “grassroots” event, which donors could watch by contributing any amount to the campaign.