Add entity event base class

src/main/java/emu/grasscutter/server/event/types/EntityEvent.java

+package emu.grasscutter.server.event.types;
+
+import emu.grasscutter.game.entity.GameEntity;
+import emu.grasscutter.server.event.Event;
+
+/**
+ * An event that is related to entity interactions.
+ */
+public abstract class EntityEvent extends Event {
+    protected final GameEntity entity;
+
+    public EntityEvent(GameEntity player) {
+        this.entity = player;
+    }
+
+    public GameEntity getEntity() {
+        return this.entity;
+    }
+}