[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