Add copytree and rmtree methods

We considered this during planning, as it could likely make use of `ops.Container.push_path` and `shutil.copytree`, or `ops.Container.remove_path` and `shutil.rmtree` without too much custom implementation needed. It seems like it will be useful for at least this project's tests -- so it could start out as a private internal helper function and get exercised that way.