Add SystemIdTemplate to allow one-shot system in bsn!

[hxYuki]

Objective

• Fixes Allow registering one-shot system in bsn! #24002

Solution

• Add SystemIdTemplate
• Implement FromTemplate for SystemId
• Add a helper function system_value<M> to convert a system to SystemIdTemplate

Testing

• A ad-hoc test nested in 3d_scene.

---

Showcase

#[derive(Component, FromTemplate)]
struct Callback(SystemId);

fn callback_scene() -> impl SceneList {
    bsn_list! {
        Callback(system_value(|| {
            println!("Hello from the callback!");
        })),
        Callback(system_value(callback_system)),
    }
}

fn callback_system(){
    println!("Hello from the system!");
}

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[hxYuki]

Not sure where to place these codes. I've searched impl FromTemplate for in repo and got only Entity here.

[hxYuki]

What should be the default value for a SystemId?