[lib][native] Add getFarcasterUsers to IdentityServiceContextProvider

Summary:
call rust client method, parse returned string, validate result

Depends on D11404

Test Plan: called this function with a set of FarcasterIDs and got back a list of FarcasterUsers

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D11405