Fix uplevel SQL Server support and differentiate External and FileTable types in object explorer list (#567)

* Differentiate External and FileTable types in object explorer list
- Add (External) and (File Table) to the labels
- Pre-fetch these properties to avoid perf hit

Note on testing: These are both quite hard to initialize, so adding tests to the integration tests would require always having a server with Polybase enabled / FileTable enabled. Given this I feel it should be OK to use manual testing. Long term, we should have unit-level tests that cover the functionality or figure out how to have a perfect pre-configured server and a set of tests to run the breadth of this logic against it.

* Fix handling of vNext servers to default to latest known version
- Default uplevel / unknown versions to the latest version
- In the IsValidFor check also just see if the server version registers as "All" since this is the default for unknown servers. This is a backup handler, the 1st fix is the primary one

src/Microsoft.SqlTools.ServiceLayer/Localization/sr.cs

@@ -2381,6 +2381,22 @@ namespace Microsoft.SqlTools.ServiceLayer
             }
         }
 
+        public static string External_LabelPart
+        {
+            get
+            {
+                return Keys.GetString(Keys.External_LabelPart);
+            }
+        }
+
+        public static string FileTable_LabelPart
+        {
+            get
+            {
+                return Keys.GetString(Keys.FileTable_LabelPart);
+            }
+        }
+
         public static string DatabaseNotAccessible
         {
             get
@@ -4639,6 +4655,12 @@ namespace Microsoft.SqlTools.ServiceLayer
             public const string SystemVersioned_LabelPart = "SystemVersioned_LabelPart";
 
 
+            public const string External_LabelPart = "External_LabelPart";
+
+
+            public const string FileTable_LabelPart = "FileTable_LabelPart";
+
+
             public const string DatabaseNotAccessible = "DatabaseNotAccessible";