For the past fifty years, David Foster produced some of music’s most iconic artists, and won 16 Grammy Awards along the way. But the hit-maker’s latest project is the Tony-nominated “Boop! The Musical,” for which Foster wrote about fifty songs. He talks with correspondent Tracy Smith about discovering such unique talents as Celine Dion and Michael Bublé; working with such demanding artists as Barbra Streisand and the rock group Chicago; and what he really thinks about legacy.