[protos][identity] Add reserved usernames to UserIdentitiesResponse

Summary:
First part of ENG-8771 - added a new response field as proposed in this comment.
This change is backwards compatible, regardless if we release identity first or clients first. The new field will simply default to an empty map.

Test Plan: Flow, cargo build. Logic is added in the next diffs

Reviewers: ashoat, varun

Reviewed By: ashoat, varun

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12922