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/ObjectExplorer/SmoModel/SmoTableCustomNode.cs

@@ -21,6 +21,14 @@ namespace Microsoft.SqlTools.ServiceLayer.ObjectExplorer.SmoModel
                 {
                     return $"{table.Schema}.{table.Name} ({SR.SystemVersioned_LabelPart})";
                 }
+                else if (table != null && IsPropertySupported("IsExternal", smoContext, table, CachedSmoProperties) && table.IsExternal)
+                {
+                    return $"{table.Schema}.{table.Name} ({SR.External_LabelPart})"; 
+                }
+                else if (table != null && IsPropertySupported("IsFileTable", smoContext, table, CachedSmoProperties) && table.IsFileTable)
+                {
+                    return $"{table.Schema}.{table.Name} ({SR.FileTable_LabelPart})"; 
+                }
             }
             catch
             {
@@ -39,6 +47,11 @@ namespace Microsoft.SqlTools.ServiceLayer.ObjectExplorer.SmoModel
                 {
                     return "Temporal";
                 }
+                // TODO carbon issue 3125 enable "External" subtype once icon is ready. Otherwise will get missing icon here.
+                // else if (table != null && IsPropertySupported("IsExternal", smoContext, table, CachedSmoProperties) && table.IsExternal)
+                // {
+                //     return "External";
+                // }
                // return string.Empty;
 
             }

src/Microsoft.SqlTools.ServiceLayer/ObjectExplorer/SmoModel/TreeNodeDefinition.xml

@@ -79,8 +79,10 @@
       </Filter>
     </Filters>
     <Properties>
+      <Property Name="IsFileTable" ValidFor="Sql2012|Sql2014|Sql2016|Sql2017"/>
       <Property Name="IsSystemVersioned" ValidFor="Sql2016|Sql2017|AzureV12"/>
 	    <Property Name="TemporalType" ValidFor="Sql2016|Sql2017|AzureV12"/>
+      <Property Name="IsExternal" ValidFor="Sql2016|Sql2017|AzureV12"/>
     </Properties>
     <Child Name="SystemTables" IsSystemObject="1"/>
   </Node>