[identity] db method to get users for a set of farcaster ids

Summary:
we need this method to implement the GetFarcasterUsers RPC

Depends on D11400

Test Plan: Registered some wallet and password users with FIDs, confirmed that their corresponding user info was returned by the db method when called with the same FIDs

Reviewers: bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11401