INT-1197: Add support for WebSockets: client side

JIRA: https://jira.spring.io/browse/INT-1197

**DO NOT MERGE YET**

For me: this PR needs JavaDocs only.
All others (namespace, server, docs etc.) might be covered within other JIRAs.

@rstoyanchev, review this, please, too.
Remove unused imports - also `WebSocketOutboundMessageHandlerTests`