[Chore]: add `aws-durable-execution-sdk-python-testing` package to `aws-durable-execution-sdk-python` repo

[zhongkechen]

aws-durable-execution-sdk-python is now a monorepo that consists of multiple Python packages. We can add aws-durable-execution-sdk-python-testing to aws-durable-execution-sdk-python repo and deprecate this github repo.