Add Send trait constraint to return type of trait functions that impl Future

[madeleyneVaca]

https://blog.rust-lang.org/inside-rust/2023/05/03/stabilizing-async-fn-in-trait.html
Using ^ as reference, make sure to add Send support for async functions to avoid future dev problems with different executors

[magravel]

I investigated adding Send to async functions in that repo, and it made the integration quite hard, because the types that impl or that are used in these functions need to be Send. Some of the fixes are to change the Nop mutex for a "real" one, which will add runtime work unnecessarily in a single-threaded executor, and other changes are to modify how some structs store things, use of Cell and raw pointer, or any !Send trait result in a type !Send. After reading a bit more of the reference above, it seems that the solution would be to have a trait that is not Send and a copy of this trait that is Send and use the one we need depending on whether we, the user of this crate, are in a multithreaded environment or not. It looks like the crate trait-variant can be used to solve this issue.