Add jspahrsummers/objc-build-scripts

This allows [Janky](https://github.com/github/janky) to simply invoke `script/cibuild`, which is its default behavior. No more project-specific configs!

Although it looks like there are a lot of changes here, most of them came via subtree merge from the [objc-build-scripts](https://github.com/jspahrsummers/objc-build-scripts) repository.