Implement HolderSet class and cleanup item codec

protocol/src/main/java/org/geysermc/mcprotocollib/protocol/data/game/item/component/AdventureModePredicate.java

@@ -16,8 +16,7 @@ public class AdventureModePredicate {
     @Data
     @AllArgsConstructor
     public static class BlockPredicate {
-        private final @Nullable String location;
-        private final int @Nullable [] holders;
+        private final @Nullable HolderSet blocks;
         private final @Nullable List<PropertyMatcher> properties;
         private final @Nullable NbtMap nbt;
     }

protocol/src/main/java/org/geysermc/mcprotocollib/protocol/data/game/item/component/HolderSet.java

@@ -0,0 +1,41 @@
+package org.geysermc.mcprotocollib.protocol.data.game.item.component;
+
+import lombok.Data;
+import lombok.NonNull;
+import org.checkerframework.checker.nullness.qual.Nullable;
+
+import java.util.function.Function;
+
+/**
+ * Represents a set of holders that could either be explicit, or resolved from a tag location.
+ * The client has to know how to resolve the tag location to get the holders.
+ */
+@Data
+public final class HolderSet {
+    private final @Nullable String location;
+    private final int @Nullable [] holders;
+
+    public HolderSet(int @NonNull [] holders) {
+        this.location = null;
+        this.holders = holders;
+    }
+
+    public HolderSet(@NonNull String location) {
+        this.location = location;
+        this.holders = null;
+    }
+
+    /**
+     * Return either the explicit holders, or resolve the tag location to get the holders.
+     *
+     * @param tagResolver The function to resolve the tag location to get the holders.
+     * @return The holders.
+     */
+    public int[] resolve(Function<String, int[]> tagResolver) {
+        if (holders != null) {
+            return holders;
+        }
+
+        return tagResolver.apply(location);
+    }
+}