[lib] define Tunnelbroker API in Flow types

Summary:
Adding Rust's types to JS codebase.

We have right now fragmentary types in tunnelbroker-messages.js but this file will be removed, after new shared interface will be implemented.

Test Plan: Flow

Reviewers: michal, bartek, varun

Reviewed By: michal

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9596