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/AdminGump.cs

@@ -74,13 +74,13 @@ namespace Server.Gumps
         private readonly AdminGumpPage m_PageType;
         private readonly object m_State;
 
+        public override bool Singleton => true;
+
         public AdminGump(
             Mobile from, AdminGumpPage pageType, int listPage = 0, List<object> list = null, string notice = null,
             object state = null
         ) : base(50, 40)
         {
-            from.CloseGump<AdminGump>();
-
             m_From = from;
             m_PageType = pageType;
             m_ListPage = listPage;