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/IEditTableMetadata.cs

@@ -16,7 +16,7 @@ namespace Microsoft.SqlTools.ServiceLayer.EditData
         /// <summary>
         /// All columns in the table that's being edited
         /// </summary>
-        IEnumerable<EditColumnWrapper> Columns { get; }
+        IReadOnlyList<EditColumnWrapper> Columns { get; }
 
         /// <summary>
         /// The escaped name of the table that's being edited
@@ -31,6 +31,6 @@ namespace Microsoft.SqlTools.ServiceLayer.EditData
         /// <summary>
         /// Columns that can be used to uniquely identify the a row
         /// </summary>
-        IEnumerable<EditColumnWrapper> KeyColumns { get; }
+        IReadOnlyList<EditColumnWrapper> KeyColumns { get; }
     }
 }