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/Contracts/EditCreateRowRequest.cs

@@ -19,6 +19,12 @@ namespace Microsoft.SqlTools.ServiceLayer.EditData.Contracts
     /// </summary>
     public class EditCreateRowResult
     {
+        /// <summary>
+        /// Each element corresponds to the default value for the column. If null, no default is
+        /// defined. Calculated columns will have placeholders provided here.
+        /// </summary>
+        public string[] DefaultValues { get; set; }
+
         /// <summary>
         /// The internal ID of the newly created row
         /// </summary>