build: an option per component, cth links boost-pfr

[codethinki]

Summary

• Every component behind an option through cth_add_subdirectory (cth_cmake#7): CTH_BUILD_OS, CTH_BUILD_CORO, CTH_BUILD_NET (default ON) and CTH_BUILD_WIN (default WIN32). A parent project turns components off with set(CTH_BUILD_<COMPONENT> OFF) before add_subdirectory(cth); the package only contains the built components.
• The components assert what they need: coro and net need cth_os, cth_win needs cth_os, cth_coro and a windows target.
• fix: the cth target links boost-pfr. cth/hash/hash_aggregates.hpp includes boost/pfr.hpp, but neither the target nor the package declared it, consumers had to find boost_pfr themselves.
• README: subdirectory usage, the options, the missing net component.
• Bumps lib/cth_cmake to 09a6c5d (refactor(data): remove the virtual destructor of raw_poly_vector #6, build: link stdc++exp through a generator expression #7).

For cth_engine, which adds cth from a submodule with coro, net and win off.

Test plan

• gcc debug, all components: 264/264
• gcc debug, CTH_BUILD_NET=OFF CTH_BUILD_CORO=OFF: 192/192, installed package only has cth and os
• CTH_BUILD_OS=OFF with coro on fails the configure with the assert
• cth_engine with add_subdirectory(lib/cth): 273/273 on gcc and clang, no install on build
• CI: Linux gcc/clang, Windows msvc/clang