Support sender-receiver model

[simmplecoder]

Notice
If you need support or clarification regarding the usage of Drogon in your project, visit the official Drogon support channel at gitter

Please create a new issue only if you think you have found a bug or if have a feature request/enhancement.

Is your feature request related to a problem? Please describe.

I would like to use Drogon with upcoming std::execution in C++26. There are two (libunifex, stdexec) libraries that very closely mimic the upcoming library features.

Describe the solution you'd like
I would like the async functionality of Drogon to support sender-receiver model described in P2300 and if possible, support one of the semi-reference libraries suggested above.

Describe alternatives you've considered
I have tried to implement a scheduler for WebSocketClient's event loop, but the proposal lacks good examples on how to retrofit old async interfaces on sender-receiver well, without ping-ponging senders around and avoiding allocations like in normal composition of senders.

Additional context
Should I open this in TRANTOR first?