[native] implement 'FindUserIdentities' RPC in native_rust_library

Summary:
This exposes FindUserIdentities to C++ through native rust library

Depends on D11952

Test Plan: Tested rpc calls on client later in stack

Reviewers: kamil, bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11998