C++ 20 coroutines for async requests

[jbaldwin]

Integrate with a coroutine library or implement a small enough subset of c++20 coroutines to allow for co_await'ing async requests.

• Use library is probably easier but adds another dependency
• Implement small subset is harder but will remain self sufficient.

It would be smart to also allow for the event loop to be driven by a user land thread to integrate with existing frameworks like seastar, etc..

It might be possible to implement this in a way that allows for conditional compliation to enable to feature, requries C++20 and the external library (say seastar) to be available to enable coroutines.