[graphiql] SSE support for subscriptions ?

Hi,

Thank you for the work.
I just implemented subscription with SSE (Server Side Event) on my server but it seems that it's not support or I didn't find any documentation on that matter..

I hoped to have an option on the [fetch](https://github.com/graphql/graphiql/blob/main/packages/graphiql-toolkit/docs/create-fetcher.md) but it seems that it only supports websockets ?

Would it be possible for GraphiQL to support SSE as well?