Initial commit of Cxx API snapshots

Summary:
This diff introduces initial snapshots of the Cxx public API in a form of a couple .api files, namely:

- ReactAndroidDebugCxx.api
- ReactAndroidReleaseCxx.api
- ReactAppleDebugCxx.api
- ReactAppleReleaseCxx.api
- ReactCommonDebugCxx.api
- ReactCommonReleaseCxx.api

that is for `ReactAndroid`, `ReactApple`, and shared `ReactCommon` in two variants `Release` and `Debug` each.

Differential Revision: D97296573

Author: Dawid Małecki