Forward cli serve settings and make it easier to provide platform config to fullstack

[ealmloff]

This PR forwards settings from dx serve to fullstack. This makes the port and open arguments work with fullstack applications. Closes #2380

It also makes it slightly easier to provide platform specific context to fullstack. You can now provide platform specific configs, and fullstack will combine them into the fullstack config automatically:

LaunchBuilder::new()
        .with_cfg(web!(
            dioxus::web::Config::new().with_default_panic_hook(false)
        ))
        .with_cfg(server_only!(dioxus::fullstack::ServeConfig::builder()
            .incremental(IncrementalRendererConfig::default())))
        .launch(app)

Stacked on #2517, #2515