Missing proxy configuration support in WebSocket API

Currently there is no spec defined mechanism for providing proxy configurations when ClientEndpoint is behind a http proxy. There are multiple variations of implementations but still such configuration could be interfaced in spec defined way so that connectToServer methods in WebSocketContainer respect that.
#### Affected Versions
[1.0]

Start by reviewing the WebSocket API specification and the WebSocketContainer connectToServer methods, focusing on how a ClientEndpoint behind an HTTP proxy could provide configuration. Compare the stated implementation variations and define a spec-level approach that those methods respect; done means the proxy configuration mechanism is specified consistently.

Written by the indexing model from the issue text.