[keyserver] opaque-ke-napi scaffolding

Summary:
Yet another dependency... this one at least is just replacing neon, which ended up not working out very well.
napi-rs does the same thing as neon (creates an addon for Node) but with much better ergonomics. It also supports tokio
by default, which lets us just use tonic for gRPC instead of the IMO harder to use Node gRPC client.

napi-rs also handles all of the typing for us so we don't get any flow issues.

Test Plan: imported the addon in keyserver.js and successfully called the sum function

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D5795