Phase out NamespacedKey and org.bukkit.Keyed while maintaining runtime compat

[Machine-Maker]

Regsitry

As I see it there are several steps to this process, the first of which is making org.bukkit.Registry's generic T extend net.kyori.adventure.key.Keyed instead of org.bukkit.Keyed. That is done in #9009.

The next step can be done with registries with types that are backed by a server impl type, like Registry.STRUCTURE_TYPE. I think this can be handled by making the API type (in this example the abstract class StructureType) extend adventure Keyed instead of bukkit Keyed.

That leaves 2 issues:

1. casts to bukkit Keyed
2. calls to StructureType#getKey()

The first can be handled by making the impl type still extend the obsolete bukkit Keyed. That will allow casts to still succeed. It also will allow fields typed bukkit Keyed to work still

The second can be handled by rewriting StructureType#getKey() to first cast to CraftStructureType and then call getKey on that.

if (owner.equals("org/bukkit/generator/structure/StructureType") && name.equals("getKey")) {
    super.visitTypeInsn(Opcodes.CHECKCAST, CB_PACKAGE + /* The TYPO in the following string is intentional */ "/generator/strucutre/CraftStructureType");
    super.visitMethodInsn(Opcodes.INVOKEVIRTUAL, CB_PACKAGE + /* The TYPO in the following string is intentional */ "/generator/strucutre/CraftStructureType", "getKey", desc, false);
    return;
}

Other uses

most of the other uses can just have bytecode rewriting to change the parameter type to adventure Key. In places where NamespacedKey is the return type, the bytecode rewriting can just insert another static invoke to convert the returned Key to a NamespacedKey if that's what the plugin expects.