[keyserver] add addReservedUsername method to rust-node-addon

Summary:
Added an identity client method to call the addReservedUsername RPC.
We will need to call this when a new account is registered on Ashoat's keyserver
via the old registration workflow to keep the Identity service aware of which
usernames should be reserved.

Depends on D8134

Test Plan: called this client method from keyserver.js with a signed message (signed by a hardcoded ed25519 key) and new username and saw that the DDB table was updated

Reviewers: jon, bartek

Reviewed By: jon, bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8201