Shared Containers (#3331)

* Namespace changes for containers.
Moved ContainerSlot from content to engine.

* Merged client/server ContainerManagerComponents into a single shared version.

* Mapfile and nullability fixes.

* Upgrades map.

* Update engine.

Content.Server/GameObjects/Components/GUI/HandsComponent.cs

@@ -457,7 +457,7 @@ namespace Content.Server.GameObjects.Components.GUI
                 throw new InvalidOperationException($"Hand '{name}' already exists.");
             }
 
-            var container = ContainerManagerComponent.Create<ContainerSlot>($"hand {_nextHand++}", Owner);
+            var container = ContainerHelpers.CreateContainer<ContainerSlot>(Owner, $"hand {_nextHand++}");
             var hand = new Hand(this, name, container);
 
             _hands.Add(hand);