[shared] introduce ping client method in grpc_clients crate

Summary: this is the method that clients will call periodically to make sure their code version is supported

Test Plan: tested in next diff by calling client method from js on native

Reviewers: bartek, michal

Reviewed By: bartek

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9812