Document new TCP proxy chaining functionality

[kenjenkins]

Let's add a section under https://www.pomerium.com/docs/capabilities/tcp#advanced-usage to document the new TCP proxy chaining support:

• Proxy chaining support for TCP routes pomerium#4362

If a TCP route has the scheme tcp:https:// in the To URL, the raw TCP connection will be proxied to the upstream. (This matches the current behavior.)

If a TCP route has the scheme http:https:// or https:// in the To URL, the HTTP CONNECT request itself will be proxied to the upstream, allowing the upstream service to terminate the CONNECT request and possibly proxy the raw TCP connection to a different backend service.