[services] Helper function to parse userID from AttributeValue

Summary:
Returns the userID String if present, otherwise returns the appropriate error

Depends on D4648

Test Plan: Tested in subsequent diff that uses helper

Reviewers: tomek, karol

Reviewed By: tomek

Subscribers: ashoat, adrian, atul, abosh

Differential Revision: https://phab.comm.dev/D4656