Add IsNullable property to EditColumnInfo class (#2545)

Author: lewis-sanchez

src/Microsoft.SqlTools.ServiceLayer/EditData/Contracts/EditColumnInfo.cs

@@ -20,5 +20,10 @@ public class EditColumnInfo
         /// such as being computed columns, identity columns, or columns that are part of a primary key.
         /// </summary>
         public bool IsEditable { get; set; }
+
+        /// <summary>
+        /// Whether or not the column allows null values
+        /// </summary>
+        public bool? IsNullable { get; set; }
     }
 }

src/Microsoft.SqlTools.ServiceLayer/EditData/EditSession.cs

@@ -510,7 +510,8 @@ public EditColumnInfo[] GetColumnInfo()
             {
                 Name = c.ColumnName,
                 // IsEditable provides a clearer public API name than IsUpdatable
-                IsEditable = c.IsUpdatable
+                IsEditable = c.IsUpdatable,
+                IsNullable = c.AllowDBNull
             }).ToArray() ?? new EditColumnInfo[0];
         }