[keyserver] APIs for finishing client side of PAKE login

Summary: using Neon to surface the Rust opaque-ke "client login finish" API and getters to Node.js

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

Reviewers: tomek, jon, bartek

Reviewed By: tomek, jon, bartek

Subscribers: bartek, ashoat

Differential Revision: https://phab.comm.dev/D5739