After seeing Stephen Colbert’s rather impressive impression of himself as Tony Stark a.k.a. Iron Man, Robert Downey Jr. was excited to offer the “Late Show” host a spot in the Avengers lineup, now that there is an opening. No, it’s not as the new Iron Man, following the death of — spoiler alert — Tony Stark in “Avengers: Endgame,” but as the assistant of another Avenger.