Support configurable connection timeout

[jprochazk]

ehttp::fetch(
    ehttp::Request::get("https://http.cat/status/200")
        .with_timeout(Some(std::time::Duration::from_secs(10))),
    move |res| { /* ... */ },
)

The timeout for all ehttp requests was always 30 seconds, due to underlying ureq defaults. This adds PR adds a way to configure it.

This is a breaking change due to adding a new field to Request, which was not previously marked #[non_exhaustive].