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.xlf

@@ -2343,6 +2343,16 @@
         <target state="new">Scripting as Execute is only supported for Stored Procedures</target>
         <note></note>
       </trans-unit>
+      <trans-unit id="External_LabelPart">
+        <source>External</source>
+        <target state="new">External</target>
+        <note></note>
+      </trans-unit>
+      <trans-unit id="FileTable_LabelPart">
+        <source>File Table</source>
+        <target state="new">File Table</target>
+        <note></note>
+      </trans-unit>
     </body>
   </file>
 </xliff>