feat(python): add package typing stubs

[abnobdoss]

What

Adds package-level typing support for the Python modules currently exposed by pyiceberg_core on main:

• pyiceberg_core.transform
• pyiceberg_core.manifest
• pyiceberg_core.datafusion

Also adds py.typed so type checkers treat the package as maintainer-typed.

Why

The runtime binding PRs intentionally avoid adding one-off .pyi files while existing Python modules are untyped. This PR centralizes typing as a package-wide concern instead.

Follow-up runtime PRs that add new Python modules can either wait for this to land and then add their own stubs in a typing follow-up, or extend this PR after their runtime APIs land.

Testing

cd bindings/python
uv run pytest tests/test_transform.py tests/test_manifest.py
# 12 passed

cargo fmt --check
git diff --check