debug_handler: Check IntoResponseParts / IntoResponse for return tuple element types #2173
Description
In Jon Gjengset's Decrusting the axum crate video, he tried using #[debug_handler] to get a more useful error message for (StatusCode, Json<_>, AppendHeaders<_>) not implementing IntoResponse as part of a handler, but that didn't work. I think we can make it work.