Support sender-receiver model

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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](https://github.com/facebookexperimental/libunifex/tree/main), [stdexec](https://github.com/NVIDIA/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](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p2300r10.html) 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?