[identity] db methods for reserved usernames table

Summary:
methods to add and remove usernames to the table, and to check if a given username is in the table

Depends on D8059

Test Plan: created a test table in localstack and successfully called all the DB client methods

Reviewers: jon, bartek

Reviewed By: jon

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8060