[keyserver] expose getFarcasterUsers client method to JS on keyserver

Summary: @atul mentioned we need to call getFarcasterUsers from the keyserver, so this code makes that possible.

Test Plan: https://gist.github.com/vdhanan/0a0498c9e672d44e0e51331c78510f80

Reviewers: atul, bartek

Reviewed By: atul, bartek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D11515