feat: Moves gumps out of the core (#1916)

> [!Important]
> **Developer Note**
> This code change will **completely move gumps out of the core**


### Summary

- Adds `GetGumps()` convenience which exposes methods to Find/Close/Send multiple gumps. This helper is a performance improvement by eliminating the Dictionary<Player, List> lookup for gumps.

Projects/UOContent/Gumps/HouseTransferGump.cs

@@ -9,6 +9,8 @@ namespace Server.Gumps
         private readonly BaseHouse m_House;
         private readonly Mobile m_To;
 
+        public override bool Singleton => true;
+
         public HouseTransferGump(Mobile from, Mobile to, BaseHouse house) : base(110, 100)
         {
             m_From = from;