[native_rust_library] Create ffi submodules for identity API

Summary:
Part of ENG-6713.

• Created a base module for Identity Service and moved first APIs there.
• Followed the convention from Backup module having the ffi submodule and further submodule tree

Depends on D11517

Test Plan: The same test plan for the whole stack: each commit compiles on both platforms. At the final commit - played a bit with these APIs, made sure they're callable from JS

Reviewers: varun, michal

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11518