[graphiql] show response headers in UI

It would be great if there was a way to show the response headers in the GraphiQL UI. This would be great for use cases where you implement stuff like [`E-Tag`](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag) and [`If-None-Match`](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag) on top of GraphQL.