Add a custom CLI to build iOS apps (#44239)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/44239

A simple CLI to build our iOS `helloworld` application.  This isn't intended for an audience other that the release team.

Some Framwork authors might be interested in it as an example of how to use our react-native/core-cli-utils to build a React Native application.

Changelog: [Internal]

I'm not going to export because it's not that interesting to folks outside / in need of scrutiny.

Reviewed By: cipolleschi

Differential Revision: D56243647

fbshipit-source-id: a5f1b6d1046bda165aa7c6848938e05f0cca2dc8