fix(python): fix OSS Cython bindings build for Linux and macOS

[Ade20boss]

Fix OSS CMake build for Python/Cython bindings

Fixes #1703

Summary

The folly/python/setup.py was incomplete, causing the OSS CMake build to fail when -DPYTHON_EXTENSIONS=ON is set. This fix brings the setup in line with what is required to successfully build and import the Cython bindings on both Linux and macOS.

Changes

• Added platform-aware library_dirs for Linux and macOS (Apple Silicon homebrew path)
• Added CMAKE_PREFIX_PATH support so the linker finds libraries built by folly's own build system, not just system paths
• Added missing library dependencies: fmt, double-conversion
• Added libunwind for Linux only — guarded with sys.platform != 'darwin' since it is not available on macOS
• Added the missing folly.fiber_manager extension
• Added missing source files: executor_intf.cpp, iobuf_intf.cpp, error.cpp
• Added include_dirs so Cython-generated code can locate folly headers
• Added -std=c++20 compile flag, required for coroutines used in folly
• Added *.pyi to package_data so type stubs are included in the install

Testing

Tested on Fedora Workstation. Additional testing was based on the investigation carried out in #1703 by @gmarzot and @novitae across Ubuntu 22.04 and macOS (Apple Silicon).

Notes

This work builds on investigation done in #1703 by @gmarzot and @novitae. The folly/python/test/ directory still needs a setup.py and test runner — that can be addressed in a follow-up.

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