[native] add getFarcasterUsers to commRustModule

Summary:
C++ code to call the new rust client method

Depends on D11403

Test Plan: Same as previous diff. We call this method from JS later in the stack and successfully get back a list of FarcasterUsers.

Reviewers: bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11404