[identity] implement client method to call identity findUserIdentities RPC

Summary: we'll need this RPC to get back a user's FID. we'll use the FID to call the neynar fetchFollowedFarcasterChannels API in the next diff and get back the list of channels that the current user is the lead in

Test Plan: https://gist.github.com/vdhanan/0936aab194560973c2ed45f65c39eaed

Reviewers: will, bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13264