Add CODEOWNERS file for all SDK repos

We should add a codeowners file to every SDK. More details about what codeowners does (and how it works) is available here: https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-code-owners

In short though, it is a way of automatically assigning reviewers or owners to pull requests and issues. This means we will less frequently encounter newly-filed issues or pull requests that are ignored, because a default recipient will be allocated (and it should be their responsibility to triage as appropriate).