feat(ext/fetch): add support for fetch on unix sockets

[lucacasonato]

This commit adds support for using Unix socket proxies in fetch API.

This is facilitated by passing an appropriate Deno.HttpClient instance
to the fetch API:

const client = Deno.createHttpClient({
  proxy: {
    transport: "unix",
    path: "/path/to/unix.sock",
  },
});

await fetch("http://localhost/ping", { client });

Closes #8821