[keyserver] APIs for starting client side of PAKE login

Summary: using Neon to surface the Rust opaque-ke "client login start" API to Node.js, along with some getters

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

Differential Revision: https://phab.comm.dev/D5738