Add support for WebSocket rules in the routing

[pgjones]

This allows for Rules to be marked as a WebSocket route and only
matched if the binding is websocket. It also ensures that when a
websocket rule is built with a scheme it defaults to the ws or wss
scheme.

[davidism]

I'm willing to get this in to 1.0.0 since it will enable Quart to depend on Werkzeug, but I'd like to see documentation along the lines of "This is for routing only, Werkzeug itself does not handle websockets. This is useful for ASGI frameworks."

[pgjones]

Great, I've added some documentation with the note/warning.

[davidism]

Looking at how new WebSocket behaves in Firefox, it requires a URL with the ws scheme and the host. So build should always build an external URL for websocket rules otherwise they won't work.

Discussing this in chat, we thought about whether WS requests should build external URLs for HTTP rules in the same way, for example in a rejection response. Since browsers assume URLs are relative and HTTP(S) unless told otherwise, we're assuming it's fine not to add this for now.