[services] add reserved usernames table to ddb

Summary: we need a table to store reserved usernames. these are usernames that have already been registered on ashoat's keyserver and therefore should not be re-assigned to a new user.

Test Plan: tested later when we actually use the table

Reviewers: jon, bartek

Reviewed By: jon

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8059