oss: add experimental-exe-ssh.py, an SSH-over-HTTPS terminal client

A dependency-less python3 client that attaches to an exe.dev VM's
terminal entirely over the HTTPS path. It signs an exe0 API key locally
with the user's SSH key, opens the per-VM xterm WebSocket with it as a
Bearer token, and wires it to the local tty in raw mode. The xterm
session is persistent, so the client auto-reconnects after drops.

Co-authored-by: Shelley <shelley@exe.dev>

Author: philz