GH-49273: [Python] Move stub docstring script to _build_utils with gr…

[vanshaj2023]

Rationale for this change

Follow-up to #49235 and #49273. The update_stub_docstrings.py script was added to MANIFEST.in to fix nightly sdist builds, but it used a fragile importlib.util.spec_from_file_location pattern and forced libcst as a hard build dependency. This gives the script a proper home and makes builds more robust for users on exotic platforms who may not have libcst.

What changes are included in this PR?

• Move scripts/update_stub_docstrings.py to pyarrow/_build_utils/update_stub_docstrings.py
• _build_utils is auto-included in sdist via existing graft pyarrow, excluded from wheels via setuptools.packages.find.exclude
• Replace importlib.util.spec_from_file_location in setup.py with a normal import wrapped in try/except ImportError
• Make libcst a soft build dependency in pyproject.toml (CI still installs it via requirements-build.txt)
• Add PYARROW_SKIP_STUB_DOCSTRINGS env var for explicit opt-out
• Keep scripts/update_stub_docstrings.py as a thin CLI wrapper

Are these changes tested?

No new tests. Verified that all modified files pass flake8, graceful degradation catches ImportError cleanly, and CI requirements files still include libcst so official wheels get docstrings.

Are there any user-facing changes?

No. Users building from sdist without libcst will now get stubs without docstrings instead of a build failure.

• GitHub Issue: [Python] Wheel / sdist build uses docstrings generation script we don't want to distribute #49273

[rok]

Thanks for working on this @vanshaj2023. I've kicked off a CI-workflow to see if it works on our CI.
That said - we have ongoing work that'll change the way we build wheels python #49357 and I have a draft proposal for docstring injection change to go with it. That'll mean #49273 might change significantly. I we probably best wait for review on #49259 to finish and then proceed with your fix on top. Feel free to review #49259 to help things move along.

[vanshaj2023]

Thanks @rok
I'll wait for #49259 to merge and then update this PR on top of it. Happy to review #49259 too.