Adding ChunkSerialization and Generation events.

[derverdox]

This patch adds new Chunk events that are called during the chunks' serialization/generation process.
Yes, there is a ChunkLoad or ChunkUnload event. However, they are called sync.

These proposed events are called during the process. They are primarily used to manipulate the persistent data container of the chunk to offramp saving/loading tasks made by custom plugins to the paper chunk loader threads system.

You may ask: Why not just store data in the persistent data container of the chunk on the fly and let paper handle the rest?
That is how it works right now, but consider a system that performs cleanup tasks to its data before storing it. You cannot do that right now because you will lag the main thread.

Having such events is an excellent alternative for those who don't want to call certain tasks sync but have to because of the synchronous nature of Bukkit's ChunkLoad / ChunkUnload events.

[github-actions]

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