Yeah, why not baseUrl, indeed. We’re just proxying to Guzzle anyway, and it could make sense for users that only want to cache one specific path (e.g., `/archive/`).

And if we call it baseUrl, I guess we can require it to contain the scheme (`my-app.com/path` looks a bit silly without scheme). Only downside is that we must throw an exception if a non-HTTP scheme is supplied, as Varnish caches only HTTP.