Added headless mode

[manuel2258]

Hey,
first, thank you for this nice plugin!

I was looking for a way to give my headless bevy game server a 'fixed tickrate' and saving and CPU usage.
This MR contains a very quick and hacked together solution, as a proof of concept.

But as already stated in issue #40, a headless mode is not really something that this plugin should support.
Therefore, my question would be whether this might still be something you would want to add to this plugin, or whether you would prefer me to fork into a specific 'bevy_servertickpace' crate, just for the server usecase?

If this is something you like to add, then I would fully clean up this MR and align the configuration part more with how it is done beforehand.

Also, do you potentially see any issues with my current implementation, especially to which Stage the system is added? I have not such an in dept knowledge about the bevy stages as you do.

[cyrilleberger]

Hi. For anyone interested, I created such a "fork" that works only for headless bevy: https://lib.rs/crates/bevy_ratepace