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.
Maybe... But I follow here with existing Rossen's code from `StompSubProtocolHandler`, where he emits events for similar messages, but send them to the channel anyway.
I see that there is no `StompCommand.CONNECTED` handling in the `handleMessageFromClient`. So, or SF should do that or we can emit that event on our own and filter such event-messages from `WebSocketInboundChannelAdapter` do not send them to the `outputChannel`.
WDYT?