Mark the Kubernetes distribution as typed

Python type checkers look for py.typed at the root of an installed package. The generated synchronous and asyncio clients mark their nested packages, but the Kubernetes distribution omitted kubernetes/py.typed, so nested model fields were treated as unknown despite precise generated annotations.

Package the root marker in source and wheel distributions, retain the existing client marker, and cover nested synchronous and asyncio model types.

Refs #2608.