Added public getter for concurrency_hint on the io_context.

[klemens-morgenstern]

This can be very useful for other libraries, as they can use it to base their own concurrency policy on it.

I specifically needs this for my async_mutex library. That is, my mutex will be much simpler for single-threaded environments, which I would like to check by querying the io_context.

[vinipsmaker]

This can be very useful for other libraries

I can confirm that. In the Lua bindings I wrote for Boost.Asio, I've created a service just to attach the concurrency hint to the io_context:

https://gitlab.com/emilua/emilua/-/blob/a166738c9f181ccce5dc6aaa92e0d2b14d141140/src/main.ypp#L375

as they can use it to base their own concurrency policy on it

For my own use, I probe the concurrency hint for the current io_context to check whether I can spawn a new thread to call run():

https://gitlab.com/emilua/emilua/-/blob/a166738c9f181ccce5dc6aaa92e0d2b14d141140/src/actor.ypp#L1855