[services][identity] helper function to get item from users table

Summary:
we have this code to get an item from the users table in a couple places, so i'm just moving it to a helper function.

Depends on D5568

Test Plan: tested in subsequent diff that uses the helper

Reviewers: tomek, bartek, jon

Reviewed By: tomek, bartek, jon

Subscribers: ashoat, atul, abosh

Differential Revision: https://phab.comm.dev/D5572