Add `mypy` presubmit to Python samples

It will be helpful to include a `mypy` presubmit to improve the end user experience and avoid type failures for users similar to: https://github.com/GoogleCloudPlatform/python-docs-samples/issues/13302.

Add a github presubmit that runs mypy on the samples to capture any type failures.

先找出儲存庫現有的 GitHub presubmit 設定，並查看 https://github.com/GoogleCloudPlatform/python-docs-samples/issues/13302 中的相關 issue。確定如何選取 Python 範例，然後確認 presubmit 會執行 mypy 並回報型別錯誤。

由索引模型根據 Issue 內容生成。