Add rapids-cython component for scikit-build based Python package builds

[vyasr]

This PR introduces various utilities that will be used across all RAPIDS Python packages that are built using scikit-build. Centralizing these utilities reduces the CMake configurations of our Python libraries to a minimum. For now, all of the content is purely CMake code, so this repo is a good place for them. If we ever add Python utilities (e.g. to simplify setup.py scripts) we may need to move to a new repo rather than polluting rapids-cmake with Python code.

[robertmaynard]

Please add .. versionadded:: v22.06.00 to the documentation in each cython/*.cmake file

[robertmaynard]

The function names need to change to match the rapids-cmake style so create_cython_modules -> rapids_cython_create_modules

[vyasr]

In addition to the inline TODOs, I have one additional question. Currently the rapids_cython_create_modules function is set up so that it will be called with modules in the same directory. The implication of that is that every Python subpackage will contain its own CMakeLists.txt. I set it up this way because that's how I've seen CMakeLists.txt files organized in projects in the past. Most RAPIDS projects seem to prefer a single monolithic CMakeLists.txt for all of the C++ code. Should I rework this function to match that pattern instead? That would entail some additional processing of file paths, but would leave us with a single listfile for all of the Python code.

[robertmaynard]

I actually have the change to move add_subdirectory(cython) inside the if(RAPIDS_CMAKE_ENABLE_DOWNLOAD_TESTS) locally, but I had told git to temporarily ignore changes to that file since I have also commented out builds of all other tests locally, so the changes weren't committed... sorry for the confusion there. Names like init are too generic for ctest -R

ctest -L cython will re-run all the tests in the cython subdirectory. If you wanted to run the init cython test you can do ctest -L cython -R init

[robertmaynard]

A couple of doc changes and we are good to go

[vyasr]

@robertmaynard I've addressed the two final requests that you had. There are also a couple of TODO items that I had questions about in the tests, create_modules/CMakeLists.txt and create_modules_with_library/CMakeLists.txt. Given that you're leaving for vacation tomorrow, though, I'm also OK with merging this PR (assuming that you're happy) and addressing these questions in a follow-up. Getting this PR merged would make it easier for me to start experimenting with this on other RAPIDS libraries while you're out.

[vyasr]

@gpucibot merge

[robertmaynard]

@gpucibot merge