Specify HTTP CONNECT implementation and extensions

We'd like to implement and recommend HTTP CONNECT as a preferred proxy mechanism in arti. Before we do so, we should document Tor's behavior in this regard as we do for SOCKS in https://spec.torproject.org/socks-extensions.html.

After we

• have the status quo documented (!392 (merged))

We can clean up its warts, including:

• Ad-hoc response codes
• Lack of onion-service informational extensions
• Lack of RFC conformance (if any)

Then we extend it as needed:

• to support Arti's RPC
• to give a supported method for providing structured isolation information (like first-party isolation)
• to set other per-connection flags, possibly
• to make sure that there's no way for a hostile website to make the browser probe the HTTP CONNECT port.
• to consider a polyglot proxy port that accepts both SOCKS and HTTP CONNECT (if this can be done safely)