`RawVulkanInitSettings` has difficulty handling common Vulkan patterns

[tychedelia]

We added support for custom Vulkan instance/device intiailization in #20565. However, it struggles to express some common Vulkan patterns.

Consider the following:

pub struct VulkanPlugin;

impl Plugin for VulkanPlugin {
    fn build(&self, app: &mut App) {
        let mut raw_vulkan_settings = app
            .world_mut()
            .get_resource_or_init::<RawVulkanInitSettings>();

        unsafe {
            raw_vulkan_settings.add_create_device_callback(
                move |mut args, adapter, additional_vulkan_features| {
                    let mut vk_sync_2_feature =
                        vk::PhysicalDeviceSynchronization2Features::default().synchronization2(true);
                    *args.create_info = args.create_info
                        .push_next(&mut vk_sync_2_feature);
                });
        }
    }
}

We want to push vk_sync_2_feature onto the pNext chain of create_info. However, because we are adding a callback, this requires that vk_sync_2_feature lives longer than callback.

In the regular open_with_callback flow used by wgpu, this is trivially avoided as the callback is immediately invoked in the course of returning the device.

[tychedelia]

This can be trivially solved by leaking a box containing the pNext member.