Add a way to configure the GraphiQL subscription endpoint

[Jannis]

The current GraphiQL interface served by the handler sends GraphQL subscriptions over HTTP, which is not expected to work. This article describes how GraphiQL can be set up with a custom fetcher that does subscriptions over a GraphQL WebSocket.

It would be great if the handler would support this to work with GraphQL servers that implement WS subscriptions.

[kivutar]

This is an example implementation from apollo-server: https://github.com/apollographql/apollo-server/blob/5d15abc681ffc43c2ce1d68d728814b8dbcf91b8/packages/apollo-server-module-graphiql/src/renderGraphiQL.ts