[keyserver] add function to grab comm userID for given fid if on keyserver

Summary:
This diff introduces the getVerifiedUserIDForFID function in a new file for farcaster utils on the keyserver.

This will be used later in the stack to determine whether or not the tagger or lead fids for a channel have comm accounts on the keyserver.

Depends on D13781

Test Plan: flow check. This was tested later in the stack. It successfully returned a userID for an fid only if the userID existed on the keyserver.

Reviewers: varun, ashoat

Reviewed By: varun

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13811