Migrate the `EntityState` code generation to ProtoData and move it to `core-java`

[armiol]

<This is a placeholder for a big umbrella-issue. It is created, so that we could reference it in PRs earlier.>

[alexander-yevsyukov]

@armiol, I think we can keep newly introduced codegen under McJava for 2.0.

We may move this code under core-java once we have the architecture for participation of libraries in custom code generation.