[services] new DB client method to get userID for a given username

Summary:
Using the username-index on the identity-users table, query for an item with the given username and return the corresponding userID

Depends on D4656

Test Plan: used client method to return a userID from DDB

Reviewers: tomek, karol

Reviewed By: tomek

Subscribers: ashoat, adrian, atul, abosh

Differential Revision: https://phab.comm.dev/D4657