[identity] [lib] [keyserver] [7/n] Add userID to reserved_usernames table in DDB

Summary:
Adding userID to reserved_usernames table. This is to enable future work streaming usernames to opensearch instance.

Depends on D10438

Test Plan: Test on local dev. Registered new user on ios and successfully persisted userID attribute in dynamoDB.

Reviewers: varun, bartek

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10480