[CommCoreModule] Introduce getCodeVersion()

Summary:
JSI codegen + implementation of getCodeVersion().

Why?
We will need access to the codeVersion on the C++ side so we can include it as an argument to the NewSession(...) unary gRPC call. To avoid defining the code version in two places (persist.js and CommCoreModule.h), we'll define it in CommCoreModule.h and access it on the JS side via this JSI function.

The next diff will replace usage of codeVersion from persist.js with a call to global.CommCoreModule.getCodeVersion()

Test Plan: Able to call getCodeVersion and log the return value

Reviewers: ashoat, def-au1t, palys-swm, varun, karol-bisztyga

Reviewed By: ashoat, palys-swm

Subscribers: Adrian, benschac

Differential Revision: https://phabricator.ashoat.com/D3138