Core/Achievements: Defined all modifier tree types and implemented many of them

Port From (https://github.com/TrinityCore/TrinityCore/commit/d0be92ec0ad0334c38b4073a7e24a571982b17da)

Source/Game/Achievements/AchievementManager.cs

@@ -59,6 +59,11 @@ namespace Game.Achievements
             return _achievementPoints;
         }
 
+        public ICollection<uint> GetCompletedAchievementIds()
+        {
+            return _completedAchievements.Keys;
+        }
+        
         public override bool CanUpdateCriteriaTree(Criteria criteria, CriteriaTree tree, Player referencePlayer)
         {
             AchievementRecord achievement = tree.Achievement;