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/SetSecureLevelGump.cs

@@ -13,6 +13,8 @@ namespace Server.Gumps
     {
         private readonly ISecurable m_Info;
 
+        public override bool Singleton => true;
+
         public SetSecureLevelGump(Mobile owner, ISecurable info, BaseHouse house) : base(50, 50)
         {
             m_Info = info;