Setup publishing for Snapshot and Stable on Maven (#34967)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/34967

This diff is a preparatory work for publishing artifacts on Maven Central.
What it does is:
1. It sets up all the 3 modules (react-native, hermes-engine, external-artifacts) for publishg
2. Adds coordinates to publish on the Snapshot repository
3. Adds support for appendign -SNAPSHOT version if invoked with `-PisNightly=true`
4. Configures GPG signing of artifacts.

I haven't touched the CircleCI and JS code yet. I'll do it in another diff.

Changelog:
[General] [Changed] - Setup publishing for Snapshot and Stable on Maven

Reviewed By: mdvacca, cipolleschi

Differential Revision: D40146212

fbshipit-source-id: 9321e16f6c18b35bc3ae785749d613085c56e7bc