Add a new event for when an entity's scoreboard tags are changed

This adds a new event, `ScoreboardTagsChangeEvent`, which is called when
an entity gets their scoreboard tags changed, through plugins calling
`Entity#addScoreboardTag`, `/tag`, `/data`, etc.

It is fired before the change happens and can be cancelled. It contains
the affected entity, which change it is happening (add, remove or
replace), and which tags are part of the change.

Author: Tmpod

paper-api/src/main/java/io/papermc/paper/event/entity/ScoreboardTagsChangeEvent.java

@@ -0,0 +1,75 @@
+package io.papermc.paper.event.entity;
+
+import org.bukkit.entity.Entity;
+import org.bukkit.event.Cancellable;
+import org.bukkit.event.HandlerList;
+import org.bukkit.event.entity.EntityEvent;
+import org.jetbrains.annotations.ApiStatus;
+import org.jspecify.annotations.NullMarked;
+import java.util.List;
+
+/**
+ *
+ */
+@NullMarked
+public class ScoreboardTagsChangeEvent extends EntityEvent implements Cancellable {
+
+    private static final HandlerList HANDLER_LIST = new HandlerList();
+
+    private boolean cancelled;
+
+    private final List<String> tags;
+    private final Change change;
+
+    @ApiStatus.Internal
+    public ScoreboardTagsChangeEvent(final Entity entity, final List<String> tags, final Change change) {
+        super(entity);
+        this.tags = tags;
+        this.change = change;
+    }
+
+    /**
+     * @return
+     */
+    public List<String> getTags() {
+        return this.tags;
+    }
+
+    /**
+     * @return
+     */
+    public Change getChange() {
+        return this.change;
+    }
+
+    @Override
+    public boolean isCancelled() {
+        return this.cancelled;
+    }
+
+    @Override
+    public void setCancelled(final boolean cancel) {
+        this.cancelled = cancel;
+    }
+
+    @Override
+    public HandlerList getHandlers() {
+        return HANDLER_LIST;
+    }
+
+    public static HandlerList getHandlerList() {
+        return HANDLER_LIST;
+    }
+
+    /**
+     * Represents the possible changes made to the entity's tags.
+     */
+    public enum Change {
+        /** The tags were added to the entity. */
+        ADD,
+        /** The tags were removed from the entity. */
+        REMOVE,
+        /** The tags replaced the entity's tags. */
+        SET,
+    }
+}

paper-server/src/main/java/org/bukkit/craftbukkit/event/CraftEventFactory.java

@@ -18,12 +18,12 @@
 import io.papermc.paper.connection.HorriblePlayerLoginEventHack;
 import io.papermc.paper.connection.PlayerConnection;
 import io.papermc.paper.event.connection.PlayerConnectionValidateLoginEvent;
+import io.papermc.paper.event.entity.ScoreboardTagsChangeEvent;
 import net.minecraft.core.BlockPos;
 import net.minecraft.core.Direction;
 import net.minecraft.network.Connection;
 import net.minecraft.network.chat.Component;
 import net.minecraft.network.protocol.game.ServerboundContainerClosePacket;
-import net.minecraft.resources.ResourceLocation;
 import net.minecraft.server.MinecraftServer;
 import net.minecraft.server.level.ServerLevel;
 import net.minecraft.server.level.ServerPlayer;
@@ -2115,4 +2115,16 @@ public static Component handleLoginResult(PlayerList.LoginResult result, PlayerC
 
         return disconnectReason;
     }
+
+    // Paper start - add ScoreboardTagsChangeEvent
+    public static ScoreboardTagsChangeEvent callScoreboardTagsChangeEvent(Entity entity, List<String> tags, ScoreboardTagsChangeEvent.Change change) {
+        final var event = new ScoreboardTagsChangeEvent(entity.getBukkitEntity(), tags, change);
+        Bukkit.getPluginManager().callEvent(event);
+        return event;
+    }
+
+    public static ScoreboardTagsChangeEvent callScoreboardTagsChangeEvent(Entity entity, String tag, ScoreboardTagsChangeEvent.Change change) {
+        return callScoreboardTagsChangeEvent(entity, List.of(tag), change);
+    }
+    // Paper end - add ScoreboardTagsChangeEvent
 }