[services] Rust Integration - c++ - Add getNumberOfCores

Summary:
Depends on D5038

We need a thread pool to address the grpc's threading problem in c++. To decide what can be a limit for the threads in the thread pool, we should probably obtain a number of CPUs on the machine.

Test Plan: services build as usual

Reviewers: jon, tomek, max, varun

Reviewed By: jon, tomek

Subscribers: ashoat, tomek, atul, abosh

Differential Revision: https://phab.comm.dev/D5112