Function export_sdl 

pub fn export_sdl<Q, M, S>(
    schema: &Schema<Q, M, S>,
    path: &str,
) -> Result<(), Error>
where
    Q: ObjectType + 'static,
    M: ObjectType + 'static,
    S: SubscriptionType + 'static,

Helper to export the GraphQL Schema (SDL) to a file for Frontend Code Generation