[native] create Client struct and add method for get_user_id

Summary: The two new functions are small enough that I've included them in the same diff. We construct a Client that holds an identity_client and then use the identity_client to call the GetUserID rpc

Test Plan: cargo build (will test more thoroughly once library is integrated into application)

Reviewers: tomek, karol, max

Reviewed By: tomek

Subscribers: ashoat, adrian, atul, abosh

Differential Revision: https://phab.comm.dev/D4733