pybind: respect CYTHON_BUILD_DIR for all Python bindings

pybind: respect CYTHON_BUILD_DIR for all Python bindings

Allow the *_processed.pyx files to be generated in a build directory
specified by the CYTHON_BUILD_DIR environment variable, instead of
always writing to the current working directory or source directory.

Changes for all bindings (rados, cephfs, rbd, rgw):
- Add cython_build_dir variable from environment
- Refactor Tempita processing to write output to build_dir
- Use os.path.relpath for the source path
- Pass cython_build_dir to cythonize() function

This ensures consistent behavior across all Python bindings and allows
for cleaner separation of source and build artifacts.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>