Neither Mozart, Verdi, Rossini, nor even the modern-day composer John Adams thought to do it, yet the age-old story was ripe for the plucking, juicy with drama and conflict, tragedy and a complicated love triangle, the thematic lifeblood of opera.  But Gerald Cohen, cantor of Shaarei Tikvah, the Scarsdale Conservative Congregation, did.  He is composing an opera based on the story of Sarah and Abraham and Hagar in the Bible’s Book of Genesis, and in doing so, he joins the ranks of American composers contributing to the new opera boom in the United States.  The first act of the composer’s opera-in-progress, “Sarah and Hagar.” will be presented in concert form at Shaarei Tikvah on Tuesday, May 24, at 8 p.m.