edit/createRow Default Values (#266)

Returns strings for the default value of a column when a new row is created. The values that could come back:
* `null` when there isn't a default for the column
* a string when there is a default for the column
* a placeholder (currently <TBD>) when the column cannot be updated

* Renaming EditTableMetadata to reflect its SMO source

* Implementation of returning default values

* Unit test for default values

* Reworking column defaults using default constraints

* Adding unit test for new sql script unwrapper

* Disabling flaky test

* Fixing oddities in tests, removing personal tests

* Fixing broken unit test

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

@@ -62,7 +62,7 @@ namespace Microsoft.SqlTools.ServiceLayer.EditData
                 throw new ArgumentOutOfRangeException(nameof(objectName), SR.EditDataObjectMetadataNotFound);
             }
 
-            return new EditTableMetadata(columns, result);
+            return new SmoEditTableMetadata(columns, result);
         }
     }
 }