[identity] db method to add username to users table in ddb
Summary:
We use the helper function for updating the users table to set the username attribute for a given user ID
Depends on D4561
Test Plan: Tested that I could set a username in DynamoDB with this method
Reviewers: palys-swm, karol-bisztyga
Reviewed By: palys-swm, karol-bisztyga
Subscribers: ashoat, Adrian, atul, yayabosh
Differential Revision: https://phab.comm.dev/D4563