[keyserver] add eth addresses to reserved usernames table

Summary: we should add eth addresses to the reserved usernames table as well so we can make sure we don't mint new user IDs for these users

Test Plan: verified eth address exists in my localstack ddb reserved usernames table

Reviewers: ashoat, atul

Reviewed By: atul

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9526