Add EntityDamageEvent and implementations

src/main/java/emu/grasscutter/server/event/entity/EntityDamageEvent.java

@@ -0,0 +1,32 @@
+package emu.grasscutter.server.event.entity;
+
+import emu.grasscutter.game.entity.GameEntity;
+import emu.grasscutter.server.event.Cancellable;
+import emu.grasscutter.server.event.types.EntityEvent;
+
+import javax.annotation.Nullable;
+
+public final class EntityDamageEvent extends EntityEvent implements Cancellable {
+    private float damage;
+    @Nullable private final GameEntity damager;
+
+    public EntityDamageEvent(GameEntity entity, float damage, @Nullable GameEntity damager) {
+        super(entity);
+
+        this.damage = damage;
+        this.damager = damager;
+    }
+
+    public float getDamage() {
+        return this.damage;
+    }
+
+    public void setDamage(float damage) {
+        this.damage = damage;
+    }
+
+    @Nullable
+    public GameEntity getDamager() {
+        return this.damager;
+    }
+}