Allow creating custom Datagrid columns (#6689)

* Change BindingTarget of DataGridBoundColumn to public, this will allow custom columns

* Changed modifier to protected

src/Avalonia.Controls.DataGrid/DataGridBoundColumn.cs

@@ -133,7 +133,7 @@ namespace Avalonia.Controls
 
         protected abstract IControl GenerateEditingElementDirect(DataGridCell cell, object dataItem); 
 
-        internal AvaloniaProperty BindingTarget { get; set; } 
+        protected AvaloniaProperty BindingTarget { get; set; } 
 
         internal void SetHeaderFromBinding()
         {