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/Localization/sr.cs

@@ -501,6 +501,14 @@ namespace Microsoft.SqlTools.ServiceLayer
             }
         }
 
+        public static string EditDataComputedColumnPlaceholder
+        {
+            get
+            {
+                return Keys.GetString(Keys.EditDataComputedColumnPlaceholder);
+            }
+        }
+
         public static string EditDataInitializeInProgress
         {
             get
@@ -1054,6 +1062,9 @@ namespace Microsoft.SqlTools.ServiceLayer
             public const string EditDataCommitInProgress = "EditDataCommitInProgress";
 
 
+            public const string EditDataComputedColumnPlaceholder = "EditDataComputedColumnPlaceholder";
+
+
             public const string EditDataInitializeInProgress = "EditDataInitializeInProgress";