Adding C++20 counting semaphore API

[hkaiser]

• adding hpx::binary_semaphore
• adding hpx/counting_semaphore.hpp that imports it into namespace hpx
• flyby: adding hpx/condition_variable.hpp and hpx/stop_token.hpp headers for
  conformance. This also imports lcos::local::conditional_variable
  into namespace hpx

[msimberg]

Thanks! Same questions as for the barrier PR though. We (I) need to go through the doxygen docs anyway before the next release so we can skip that part now as well.

[msimberg]

@hkaiser I summarized our API discussion from yesterday in #4329. Should we already go ahead and collect the "API headers" into the include module in this PR (and #4636, #4635)? I can also collect them afterwards while doing all the other headers.

[hkaiser]

Should we already go ahead and collect the "API headers" into the include module in this PR (and #4636, #4635)?

I thought we will create at least 3 include modules, one for each of the planned configurations. Should we go ahead and make that split now?

[msimberg]

Should we already go ahead and collect the "API headers" into the include module in this PR (and #4636, #4635)?

I thought we will create at least 3 include modules, one for each of the planned configurations. Should we go ahead and make that split now?

Yes, I'd say that's the plan in the end. But if we decide to skip splitting up the main library for 1.5.0 then we could still already have them in the include module. Either way. All I would like is that the headers from #4626 that we can put in place we do put in place for 1.5.0. It doesn't matter too much which module they are in before the main library is split into three.

[hkaiser]

This should be good to go now as well.

[msimberg]

Thanks @hkaiser. I just rebased this because of a conflict with #4634 in the CMake files. I'll merge this as soon as some of the GH actions builders come back clean.