Support scikit-build-core

[vyasr]

Description

This PR adds a new cython-core module of rapids-cmake that can be used with scikit-build-core. cython-core is designed as a drop-in replacement for cython, allowing consumers to simply change the include(cython) to include(cython-core) and have things generally work as expected when switching Python build backends.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.
• The cmake-format.json is up to date with these changes.
• I have added new files under rapids-cmake/
  • I have added include guards (include_guard(GLOBAL))
  • I have added the associated docs/ rst file and update the api.rst

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[bdice]

Are cython and cython-core meant to be used separately? Do we need to document that or raise an error if a project uses both? Can projects mix cython and cython-core in dependencies, like rmm using cython-core while cudf uses cython?

[robertmaynard]

Missing documentation and cmake-format updates

[vyasr]

Missing documentation and cmake-format updates

I added docs. The new functions are identical to the old ones, just included differently (include(rapids-cython-core) instead of include(rapids-cython)), so there are no new additions required for cmake-format at this stage.

[vyasr]

/merge