Support a hook for resolving self-requests

StarlingMonkey supports `fetch` calls with relative URLs, same as browsers do. In that case, the URL is resolved using the incoming request's URL as the base. E.g., when processing a request to `https://foo.com/`, the call `fetch("/bar")` will result in an HTTP request to the URL `https://foo.com/bar`.

That isn't always the desired behavior: in [some contexts](https://github.com/spinframework/spin-js-sdk/issues/298), this kind of self-request needs to be made to internal hosts to result in the correct behavior.

To address this, StarlingMonkey should provide a way to hook into the resolution. I think I have an approach in mind that should work well, but will have to think about it a bit more.