[services] create constants module for identity service

Summary: A single location for all const values in the identity service. Resolves https://linear.app/comm/issue/ENG-1174/create-constants-module

Test Plan: Checked all places where I swapped out a hardcoded value for const variable name, cargo build

Reviewers: palys-swm, karol-bisztyga, jimpo

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, atul, yayabosh

Differential Revision: https://phab.comm.dev/D4388