[native] expose client functions across FFI boundary

Summary:
This diff adds all the synchronous functions implemented in previous diffs to the ffi module. When we build the project, the C++ header file for our library will expose these functions, making them callable from our native C++ code.

Depends on D4937

Test Plan: linked the library to our android cmake project, successfully called the functions and got a response from the identity service

Reviewers: tomek, karol, atul

Reviewed By: tomek

Subscribers: ashoat, abosh

Differential Revision: https://phab.comm.dev/D4938