Allow GraphiQL headers to be specified when constructing the playground handler

[mihaip]

The GraphiQL.createFetcher function supports an optional headers argument to allow HTTP headers that are sent when doing endpoint requests (see https://github.com/graphql/graphiql/blob/main/packages/graphiql-toolkit/docs/create-fetcher.md#headers). It would be nice if there was a way to configure this when creating it, ideally passed from the playground.Handler or similar Go endpoint. While headers can be edited in the UI, for our application we generate the necessary headers programmatically already, and being able to pass them in when creating the playground handler would save the developer a copy/paste step.

Happy to submit a PR to implement this.