[keyserver] implement registerUser rpc with napi-rs/tonic/opaque-ke

Summary:
This is almost completely copy-pasted from the work I did on the native rust library.
All the differences will be annotated inline.

Test Plan: imported the registerUser function and successfully called it from keyserver.js

Reviewers: ashoat, tomek, max, bartek, jon, atul

Reviewed By: ashoat, tomek, max, bartek, atul

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D5796