[keyserver] expose APIs to start PAKE registration on client side

Summary: using Neon to surface the Rust opaque-ke library to Node.js

Test Plan: called the new APIs from a Node module and confirmed the results

Reviewers: tomek, bartek, jon

Reviewed By: tomek, bartek, jon

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D5736