[keyserver] Script to print largest Farcaster communities

Summary:
Rahul needed this for some of his work so I whipped up a quick script. Wanted to commit it to the codebase in case we need to re-run it later to get an updated list.

Details in OPS-283.

Test Plan: I ran the script. Results are in the Linear task

Reviewers: ginsu

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12020