[identity] dedup Registration and Reserved Username Registration
Summary:
a refactor to reduce some code duplication
Depends on D9526
Test Plan: successfully registered a new user with the normal registration workflow and registered an existing user with the reserved registration workflow. everything still works
Reviewers: bartek, michal, jon
Reviewed By: bartek
Subscribers: ashoat, tomek, wyilio
Differential Revision: https://phab.comm.dev/D9527