gh-142417: Add PyInitConfig_SetInitCallback() function

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Add init_callback and init_callback_arg members to PyConfig.

• Issue: Please restore _Py_InitializeMain or provide an alternate API for it #142417

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📚 Documentation preview 📚: https://cpython-previews--142420.org.readthedocs.build/

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Current documentation:

Set an initialization callback. It allows executing code as soon as the
Python interpreter is initialized, before the first import. For example, it
can be used to add a meta path importer into sys.meta_path.

Python is not fully initialized yet when the callback is called. For
example, sys.stdout may not exist yet.

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@indygreg: Hi, would such C API fit your needs for PyOxidizer?