Websocket service checks

[sylr]

Feature Description

2 new types of checks:

• Websocket Client
• Websocket Server (if possible)

Those checks would fail as soon as the service terminates the websocket or does not write anything for a configurable amount of time.

In Websocket Client consul would established the connection to the service like an HTTP check. In Websocket Server consul would run a websocket server and the service would be the one establishing the connection to the service.

[pearkes]

We've added a range of more specialized checks (such as Docker, gRPC) and would likely accept a PR for this but I given the TCP check should satisfy part of this scenario and the script check be a good fallback for more "intelligent" checks, it is unlikely we'll take on this work soon.