Add FlatStatModifier and PercentStatModifier classes; introduce Stat enum for character attributes

Assets/Scripts/Systems/CharacterModifierManager.cs

@@ -0,0 +1,29 @@
+using System.Collections.Generic;
+using Data;
+using Interfaces;
+using Sirenix.Serialization;
+using UnityEngine;
+
+namespace Systems
+{
+    public class CharacterModifierManager : MonoBehaviour
+    {
+        [OdinSerialize] private List<IStatModifier> activeModifiers = new();
+        
+        [SerializeField] private Character character;
+        
+        public void EquipItem(IStatModifier modifier)
+        {
+            activeModifiers.Add(modifier);
+            modifier.Apply(character.attributes);
+        }
+        
+        public void UnequipItem(IStatModifier modifier)
+        {
+            if (activeModifiers.Remove(modifier))
+            {
+                modifier.Remove(character.attributes);
+            }
+        }
+    }
+}