[Cosmos] Include typehints for internal files

Most of the internal methods and files in the SDK don't have proper typehinting the way our public surface area does, which makes it harder for developers to know exactly what each relevant argument is. This issue serves as a tracking item to get that documentation rolled out properly, in order to ensure that moving forward we can do better.

Ideally, the work would be divided into separate PRs on either sets of similar files, sets of similar features, or even individual files in order to more easily review - otherwise reviewing the entire SDK in one go would likely take a long time.

Follow up to comments made in https://github.com/Azure/azure-sdk-for-python/pull/41272.