[identity] get farcaster users rpc implementation

Summary:
Use the DB client method in previous diff to get the relevant user info for all users matching the farcaster IDs provided

Depends on D11401

Test Plan: called the RPC with a set of FarcasterIDs, got back user info for the FarcasterIDs that matched users in DDB

Reviewers: bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11402