[protos][identity] Add username to AuthResponse

Summary:
Address ENG-8137.

It turns out to be simple to add username to the AuthResponse on Identity side.
On proto/RPC side, this is also backwards compatible from both sides. If the response doesn't contain username field (client has newer version than deployed Identity Service), it will be defaulted to empty string.

Test Plan: Tested together with D12134 - verified AuthResponse to contain a username in all registration and login RPCs

Reviewers: ashoat, varun, will

Reviewed By: varun

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12127