Make "fetch" module public

Hi! any chance you might want to expose the [`mod fetch`](https://github.com/http-rs/http-client/blob/main/src/wasm.rs#L76) publicly or make it a tiny crate? I started doing something similar and later I just plain copy-pasted the module since it does everything I need, I'm getting some requests that I want to convert to `http_types::Request`, same with responses and potentially converting the other way around from the http_types to JS equivalent. Even the `WindowOrWorker` is useful.