Add support for WebSocket Protocol Extensions

This commits adds simple, overridable WebSocket Extension
filtering during the handshake phase and adds that
information in the WebSocket session.

The actual WebSocket Extension negotiation happens
within the server implementation (Glassfish, Jetty, Tomcat...),
so one can only remove requested extensions from
the list provided by the WebSocket client.

See RFC6455 Section 9.

Issue: SPR-10843
Update WebSocket extensions change

- add WebSocketHttpHeaders
- client-side support for WebSocket extensions
- DefaultHandshakeHandler updates
- replace use of ServletAttributes in JettyRequestUpgradeStratey
- upgrade spring-web to jetty 9.0.5
Merge pull request #392 from bclozel/SPR-10843