[RFC] Add support for C++26 std::execution

[SidneyCogdill]

Alternative to #653.

Note:

• This implementation is thread safe, but multipart message operations are not. I've experimented with multiple designs (separate send/receive socket pair; ring buffer queue; socket-wide critical section; functional style interface) in my local development project, in an attempt to fix/mitigate the hazard, but the end conclusion is basically don't do anything (as is currently implemented in this PR), as the mitigations/workarounds come with trade-offs that make them largely not viable to be used in real world scenarios, and the "don't do anything" approach significantly simplifies the implementation.
• Some comments are outdated (partly related to the experiments)
• event loop depends on Boost.Asio.
• Currently depends on stdexec for std::execution implementation. most of the primitives being used will be available in C++26.
• CPM is added.