[client-backup] extract logic for getting primary identity public key

Summary:
This code is duplicated and will be duplicated, so wrapping this in one function.

Depends on D9001

Test Plan: Verify function result is correct (SIWE login)

Reviewers: michal, marcin, bartek

Reviewed By: marcin

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9074