update filebrowser expand contract and methods (#500)

src/Microsoft.SqlTools.ServiceLayer/FileBrowser/Contracts/FileBrowserExpandedNotification.cs

@@ -17,9 +17,14 @@ namespace Microsoft.SqlTools.ServiceLayer.FileBrowser.Contracts
         public string OwnerUri;
 
         /// <summary>
-        /// Expanded node 
+        /// Expand path
         /// </summary>
-        public FileTreeNode ExpandedNode;
+        public string ExpandPath;
+
+        /// <summary>
+        /// Children nodes
+        /// </summary>
+        public FileTreeNode[] Children;
 
         /// <summary>
         /// Result of the operation

src/Microsoft.SqlTools.ServiceLayer/FileBrowser/Contracts/FileTreeNode.cs

@@ -23,7 +23,7 @@ namespace Microsoft.SqlTools.ServiceLayer.FileBrowser.Contracts
         /// <summary>
         /// List of children nodes
         /// </summary>
-        public List<FileTreeNode> Children { get; private set; }
+        public List<FileTreeNode> Children { get; set; }
 
         // Indicates if the node is expanded, applicable to a folder.
         public bool IsExpanded { get; set; }