Snapshot Inventories need to be implemented

As of 1.12.2, Spigot has introduced Inventory Snapshots in their API. The use of this function and how to implement it has to be investigated.

Sources of interest:

https://github.com/GlowstoneMC/Glowstone/blob/47db4c461f2077620731692777c07e146af4d3f1/src/main/java/net/glowstone/block/state/GlowContainer.java#L126-L128 (which is re-implemented in some classes that have custom inventory types, like `GlowFurnace`, `GlowBeacon`, etc.)

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Start with the linked GlowContainer.java implementation around lines 126-128, then inspect the custom inventory types mentioned, including GlowFurnace and GlowBeacon. Compare their behavior with Spigot's Inventory Snapshot API and determine the affected inventory classes. Done means snapshot inventory support is implemented consistently across the relevant types and verified against the API behavior.

Written by the indexing model from the issue text.