Cleanup: ExtensionCableSystem (#36052)

* Use Entity<T> and avoid Owner

* Use SharedMapSystem

Content.Server/Power/Components/ExtensionCableReceiverComponent.cs

@@ -7,7 +7,7 @@ namespace Content.Server.Power.Components
     public sealed partial class ExtensionCableReceiverComponent : Component
     {
         [ViewVariables]
-        public ExtensionCableProviderComponent? Provider { get; set; }
+        public Entity<ExtensionCableProviderComponent>? Provider { get; set; }
 
         [ViewVariables]
         public bool Connectable = false;