Refactor XML websocket namespace

- websocket:handler-mappings is now websocket:handlers
- some external elements are now subelements

Issue: SPR-11063
Fix typo
WIP XML websocket namespace

- added bean definitions for messageConverter, messagingTemplate,
annotationMessageHandler
- register messageBroker (simple, noop or relay)
- refactor XML attribute names to lowercase and "-"

Issue: SPR-11063