[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