Add MVP `std::execution` qthreads scheduler with hello world test

[msimberg]

Not ready for merging (if ever, perhaps this should be outside the repo?), just for discussion/testing.

Build with

./configure --enable-cxx-tests CXXFLAGS="-std=c++20 -I/path/to/stdexec/include -pthread" LDFLAGS="-lstdc++"

[insertinterestingnamehere]

Yah, I'm fine with including this in the main repo. I think the open questions are just how we're doing our branching and what the default C++ build config should be. It's a useful point of reference though so it'd be great to include it in some form.

[janciesko]

I would like to propose to continue the development of the stdexec ult examples here https://github.com/janciesko/kokkos-stdexec-examples. Once we're happy, we should move the repo to Kokkos.org.