Improve HPX package management of coroutines implementation (#17654)

* introduce logic for boost+context dependency and generic_context variant

* fix OTF2 instrumentation minor problem

* default coroutine impl depends on platform

* fix flake8

* add reference to ~generic_coroutines conflict info

* Update var/spack/repos/builtin/packages/hpx/package.py

Co-authored-by: Adam J. Stewart <ajstewart426@gmail.com>

Co-authored-by: Adam J. Stewart <ajstewart426@gmail.com>