[services][identity] update put_token_helper to return a String
Summary:
This change lets us reuse this helper function for the PAKE registration workflow. Previously, this function returned a struct specific to the Login workflow.
Depends on D4360
Test Plan: cargo build
Reviewers: jimpo, palys-swm, karol-bisztyga
Reviewed By: palys-swm
Subscribers: ashoat, Adrian, atul, yayabosh
Differential Revision: https://phab.comm.dev/D4361