fix: Cleans up entity persistence. Generalizes Mobiles/Items/Guilds (#1528)

### Summary
- [X] Fixed a bug where entity persistence was serialized out of order, causing world corruption.
- [X] Fixed LastSerialized not being utilized properly and dangling references still becoming an issue.
- [X] Added a new `GenericEntityPersistence<T>` type to encapsulate `ISerializable` serialization.
- [X] Removing the custom logic and moved Items, Mobiles, Guilds, and Accounts  to GenericEntityPersistence.
- [X] Changed serialization to use the singleton pattern to reduce calling methods from stored variables.

Projects/UOContent/Misc/Titles.cs

@@ -321,7 +321,7 @@ namespace Server.Misc
 
             if (beheld is PlayerMobile mobile && mobile.DisplayChampionTitle)
             {
-                var titleLabel = mobile.GetChampionTitleLabel();
+                var titleLabel = ChampionTitleSystem.GetChampionTitleLabel(mobile);
                 if (titleLabel > 0)
                 {
                     // Should this be translated to the receivers language? Prefix titles aren't?