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

Summary:
Added an identity client method to call the removeReservedUsername RPC.
We will need to call this when an account is deleted on Ashoat's keyserver to keep the Identity service aware of which usernames should be reserved.

Depends on D8201

Test Plan: called this client method from keyserver.js with a signed message (signed by a hardcoded ed25519 key) and an existing reserved 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/D8202