A follow up to #1214. Add support for CONNECT to envoy. There are 2 use cases for CONNECT that I'd like to add. Envoy receiving a CONNECT request, connecting to an upstream, issuing a 200 and then downgrading to L4 (similar to websocket support). The second is envoy using CONNECT when initiating an upstream L4 connection (the moral equivalent of envoy using http_proxy for some https upstreams). I think the first one is pretty straightforward.

• Add a config option to enable CONNECT on the listener in http1_options
• Update codec to support the right authority in CONNECT
• Write a Http11ConnectHandlerImpl in the spirit of WsHandlerImpl

The second case is a little more unclear to me. Here's a straw man.

• Upstreams get a 'use connect' flag.
• When enabled any connections to that upstream are initiated with a CONNECT request exchange before the remain traffic is forwarded.