Add Support for Custom Scheduler

[Excel619]

The ACFBukkitScheduler/ACFPaperScheduler/ACFFoliaScheduler classes all have the same method signatures.
ACF should support injecting a custom scheduler which is responsible for handling asynchronous execution.

This is particularly useful when working with the Kotlinx coroutines library, or other custom scheduling that doesn't rely entirely on the Bukkit/Paper scheduling APIs.