[identity] introduce RPC to update reserved usernames list

Summary: when accounts are created/deleted on ashoat's keyserver, it will need to inform the identity service so that the list of reserved usernames can be updated.

Test Plan: will be tested when implemented on identity service

Reviewers: ashoat, jon

Reviewed By: ashoat, jon

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8133