Consider C++20 coroutines support

[drewdzzz]

We could support required operators and introduce a new method like wait that integrates with C++20 stackless coroutines - it would be enabled only if C++20 (or more modern standard) is used.

[CuriousGeorgiy]

We partially touched on this in #78: I think we came to a conclusion that this is out of the scope of our library, and we should only provide the bare minimum convenient interfaces to build any execution model on top.

I would again invite @Gerold103 to such a discussion and piggyback on his experience and serverbox.

[Gerold103]

C++20 coroutines the users could just add on their own if your client has any sort of "callback" to attach to each request. Because C++20 coroutines are basically "resumable functions". Callbacks that you can continue and pause. They are isomorphic.

In serverbox it works so easy exactly because my main execution unit there was always a callback. An std::function. Coroutines can literally be wrapped into an std::function.