Passing options when sending fetch requests

Some time ago we chatted how we could make it possible to pass [request options](https://developer.mozilla.org/en-US/docs/Web/API/fetch#syntax) to `fetch.send()`. I actually forgot about the issue until I hopped back on my toy project this weekend and got stuck 😆 Opening a GH issue this time to keep it visible.

[Here's the related discord discussion](https://discord.com/channels/768594524158427167/768594524158427170/1115017724658208898). We didn't reach any conclusion but it should help jog our memory.

The simplest solution would be to introduce a `FetchOptions` type that we would pass to a new dedicated function like `fetch.send_with_options(request, options)`.

Another path would be having a new `FetchRequest` type that wraps the classic `gleam/http/request.{Request}` with additional fetch-specific fields, such as `mode`, `credentials`, etc. We could then provide a builder pipeline to take the request, add options, and send it.