fix: Fixes stabled, abilities targeting self, and unsummon memory leak (#1418)

## **MAJOR CHANGE**
* `Stabled` has been moved to `PlayerMobile`.
* New methods added, `PlayerMobile.AddStabled` and `PlayerMobile.RemoveStabled`.
* Added `PlayerMobile.AddFollower` and `PlayerMobile.RemoveFollower`.
* `Stabled`, `AutoStabled`, and `AllFollowers` are now `HashSet` and **_CAN BE NULL_**.

### Summary
* Fixes monster abilities causing harm to the monster through reflect
* Adds `CanTriggerAgainstSelf` to override this for healing or some other self-affecting ability
* Fixes a major memory leak where `UnsummonTimer` from animated dead spell lasts up-to 24hrs and therefore holds onto references of dead/deleted mobs.
* Fixes another minor leak where a mob is not unregistered from the animated dead spell list until the next spell cast.

Projects/Server/Serialization/SerializationExtensions.cs

@@ -48,10 +48,18 @@ public static class SerializationExtensions
         return entity?.Created <= reader.LastSerialized ? entity : null;
     }
 
-    public static List<T> ReadEntityList<T>(this IGenericReader reader) where T : class, ISerializable
+    public static List<T> ReadEntityList<T>(
+        this IGenericReader reader,
+        bool nullIfEmpty = false
+    ) where T : class, ISerializable
     {
         var count = reader.ReadInt();
 
+        if (count == 0 && nullIfEmpty)
+        {
+            return null;
+        }
+
         var list = new List<T>(count);
 
         for (var i = 0; i < count; ++i)
@@ -66,10 +74,18 @@ public static class SerializationExtensions
         return list;
     }
 
-    public static HashSet<T> ReadEntitySet<T>(this IGenericReader reader) where T : class, ISerializable
+    public static HashSet<T> ReadEntitySet<T>(
+        this IGenericReader reader,
+        bool nullIfEmpty = false
+    ) where T : class, ISerializable
     {
         var count = reader.ReadInt();
 
+        if (count == 0 && nullIfEmpty)
+        {
+            return null;
+        }
+
         var set = new HashSet<T>(count);
 
         for (var i = 0; i < count; ++i)