Websocket: re-enable sockets via proxy block

[malud]

Background
Currently the websocket protocol upgrade is not supported in favour of the option for multiple proxy / request calls within an endpoint. The Upgrade Response will not Hijack the current client / backend connection to pipe the bytes.

Proposal
Since the easiest and common way is a 1:1 client <> backend relation which should be upgraded while proxying. To do so we need to know which proxy block is enabled / allowed to do the upgrade. The backend block would be not a good place for this option, since multiple possible references leads to the same issue: which backend response will do the upgrade?

Introducing websocket: true to the proxy block:

proxy {
  websocket = true
  backend {
    # ...
   }
}

Other requests MAY be canceled on upgrade and a response block should not answer on upgrades.

Configuration validation should ensure proper endpoint websocket usage.