Add core cython hello

[jcfr]

Once finalized, this pull request will add the core-cython-hello sample project demonstrating how to leverage scikit-build-core and cython.

In the meantime, it is used as a "maturing" ground to consolidate the changes originally developed by @vyasr in the context of rapidsai/rapids-cmake#433

Related issues:

• Support editable installs of packages containing Cython declaration files scikit-build-core#442
• [ENH] Add CMake module for compiling Cython code cython/cython#5486
• https://gitlab.kitware.com/cmake/cmake/-/issues/24982
• Support scikit-build-core rapidsai/rapids-cmake#433

[vyasr]

@jcfr you can start using https://github.com/vyasr/cython-cmake/ in this example. The code there is ready for some initial stress testing.

[jcfr]

The failures are associated with the pi-fortran sample project and are not related to the core-cython-hello project that is successfully built and tested.