A callback that would allow us to set the chance for a boss to be champion

[Foks256]

There currently isn't any easy way to force a boss champion (there is morph yet that may cause issues), it would be really cool to have a callback in which returning a number between 0-1 would dictate their spawn chances, so 0 would be no champion bosses spawn, 1 would be they always spawn, it would also be cool to force a specific boss champion to appear.