[graphiql] Sending extensions in request

Hello, it can be useful to send extensions in a GraphQL request and I'd like to know if you're interested in supporting this feature.

At work, I have a use case where I want to send metadata via the extensions key in a GraphQL request. This metadata is then used to toggle behaviour in the GraphQL service, which is very useful for developers wanting to quickly iterate and test ideas in staging.

I saw a [previous discussion about request extensions](https://github.com/graphql/graphiql/issues/375) from 2017. Given that GraphiQL has changed significantly since then, I thought it could be easier to implement this now.

Would you consider this as a feature? I am happy to help out.