[keyserver] more APIs for finishing client side of PAKE registration

Summary: using Neon to surface the Rust opaque-ke "client registration finish" APIs 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: bartek, ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D5737