Move `codegen-plugins` to `master` branch

[Oleg-Melnik]

This PR moves codegen-plugins Gradle project to the master branch.

The structure of codegen-related modules is changed and currently it looks like the following:

— Chords

• |— codegen
• • |— runtime — Chords module with MessageField, MessageOneof, and MessageDef interfaces.
• • |— plugins — separate Gradle project with ProtoData plugins.
• • |— workspace — git submodule that refers codegen_workspace branch with separate Gradle project.
• • |— tests — Chords module where codegen-plugins are applied and tests are executed.