[services] [1/n] Add username to KeyserverKeysResponse

Summary: This adds an identity subfield to KeyserverKeysResponse. This allows verification of keyserver username through identity service.

Test Plan: Made a getKeyserverKeys grpc request and confirmed keyserver info + username response

Reviewers: varun, bartek, tomek

Reviewed By: varun, bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10586