Add react-native-test-library package (#43068)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/43068

This diff adds `react-native-test-library` package.
It contains native module and native component example, and targets both the new and the old architecture. It has structure similar to many OSS React Native libraries, and is supposed to be used to test the integration with third-party libraries.

It is integrated with RNTester as the **OSS Library Example** screen.

{F1457510909}

**Change Background** tests native commands.
**Set Opacity** tests native props.
**Get Random Number** tests native module.

Changelog: [Internal]

Reviewed By: RSNara

Differential Revision: D50793835

fbshipit-source-id: ff6daefab10e6e9f13049e3013f8f63cfa8a929e