PEP-561 & PEP-484 style type support

It would be really great to add type annotations to this module so that downstream consumers don't need to either disable it from their type checkers, or do their own limited-scope stubbing.

There's some indication that there's demand (other than mine) from this stackoverflow: https://stackoverflow.com/questions/57785471/why-does-mypy-think-library-imports-are-missing

I'm more than up for helping out with this is this something the maintainers are interested in, otherwise typeshed is a potential alternative (though keeping it inline would be better for maintainability by far!)

I searched the issues as best I could and I don't believe this is duplicate, so at the very least this can act as a good gauge for how many people are interested in this as a feature!

この issue では、変更が必要なモジュール、テスト、エントリーポイントが特定されていません。まず影響を受ける SDK モジュールを把握し、そのパッケージメタデータがどのように型を公開しているかを確認してから、アノテーションの範囲と検証アプローチについてメンテナーと合意します。下流の型チェッカーがローカルスタブや除外なしでモジュールを利用できれば完了です。

索引モデルが issue の本文から書いたものです。