Websocket support in HTTP service

[elasticmachine]

Original comment by @kjbekkelund:

Canvas is exploring websockets, so it's something we might want to think about in the context of the new platform.

(This is just a placeholder after discussing with @w33ble)

[lukasolson]

Right now, we're doing some pretty funky stuff to get at the auth headers so that we can create an authenticated Elasticsearch client the socket can use. See https://github.com/elastic/kibana-canvas/issues/451.

[stacey-gammon]

Let's close this since it's not a short term goal anymore and we removed websockets from canvas. We can reopen when we need to explore bringing it back in.