Add websocket url support for docs

[vladshcherbin]

Is your feature request related to a problem? Please describe.
I have a server with only websockets, for viewing docs in altair an http link has to be passed.

Describe the solution you'd like
Would be great if I could choose ws from dropdown list, set a ws link as URL and docs would automagically appear 🎉

Additional context
After asking this in graphql chat, graphiql has support for custom fetcher. I've tried suggested gist with websocket fetcher, added my websocket url link and docs appeared ✨