internal/dag: add websocket support to ingressroute

[alexbrand]

Adds a new field to the Routes object inside the IngressRoute CRD called EnableWebsockets, which controls whether a client can upgrade to a websocket connection on the given route.

Fixes #595

[stevesloka]

Overall this LGTM with a couple comments:

1. Is there any reason to not default this on for all routes?
2. It might be nice to as well have a default like we do for HealthChecks / LBAlgo to set it at the root and let it cascade down (probably as a new issue).

[alexbrand]

Thanks @stevesloka. You raise a good question in number 1. My assumption is that we default websockets to be disabled because Envoy does so. I did some research and found some context around this in the envoy repo: (envoyproxy/envoy#319 (comment)):

We should explicitly specify in the route configuration whether a route is a WS route or not. Meaning, we will not support attempting to detect if it is WS all the way back to the backend. If we receive a WS upgrade over HTTP/1.1, we will check the route table to see if the matched route is WS, and if not, fail the request with 400 or some other code.

I still don't quite understand the reasoning behind having to mark a route as WS though. Curious if @dfc has any thoughts here.

[davecheney]

LGTM. Thank you.