refactor(python): Separate schema cython classes into their own module (

#558)

This PR separates out the schema-related Cython classes into their own
file as part of a larger effort to improve the scope, readability,
documentation, and IDE support for nanoarrow/Python's internals.

This PR:

- Moves the Schema code
- Adds basic docstrings to internal functions that were missing

python/setup.py

@@ -139,6 +139,7 @@ def nanoarrow_extension(
         nanoarrow_extension("nanoarrow._buffer", nanoarrow_c=True),
         nanoarrow_extension("nanoarrow._lib", nanoarrow_c=True, nanoarrow_device=True),
         nanoarrow_extension("nanoarrow._ipc_lib", nanoarrow_c=True, nanoarrow_ipc=True),
+        nanoarrow_extension("nanoarrow._schema", nanoarrow_c=True),
     ],
     version=version,
 )