Allow IInventoryService.GetFolder(folderId, userId) as well as GetFolder(InventoryFolderBase folder)

This involves no wire changes since the methods often just construct an InventoryFolderBase under the hood.
This is in line with other methods that alraedy allow requests via uuid

OpenSim/Tests/Common/Mock/MockInventoryService.cs

@@ -162,6 +162,11 @@ namespace OpenSim.Tests.Common.Mock
         {
             return null;
         }
+        
+        public InventoryFolderBase GetFolder(UUID folderId, UUID userId) 
+        { 
+            return null; 
+        }
 
         public InventoryFolderBase GetFolder(InventoryFolderBase folder)
         {