ci: add a pipeline for consuming react-native-macos

[tido64]

Please select one of the following

• I am removing an existing difference between facebook/react-native and microsoft/react-native-macos 👍
• I am cherry-picking a change from Facebook's react-native into microsoft/react-native-macos 👍
• I am making a fix / change for the macOS implementation of react-native
• I am making a change required for Microsoft usage of react-native

Summary

Add a pipeline for consuming react-native-macos outside of the repository. This should help catch issues earlier (such as #860 or FBReactNativeSpec not being generated).

Backports #878

Changelog

[Internal] [Added] - Add a pipeline for consuming react-native-macos outside the repository

Test Plan

The newline pipeline should successfully build react-native-macos.

[tido64]

@Saadnajmi Can we merge this without having to pass the CircleCI tests? They don't run and I'm pretty sure they're unrelated.

[Saadnajmi]

@Saadnajmi Can we merge this without having to pass the CircleCI tests? They don't run and I'm pretty sure they're unrelated.

@HeyImChris is the one who can do that