Allow statistics to be collected for foreign tables.

ANALYZE now accepts foreign tables and allows the table's FDW to control
how the sample rows are collected.  (But only manual ANALYZEs will touch
foreign tables, for the moment, since among other things it's not very
clear how to handle remote permissions checks in an auto-analyze.)

contrib/file_fdw is extended to support this.

Etsuro Fujita, reviewed by Shigeru Hanada, some further tweaking by me.

doc/src/sgml/maintenance.sgml

@@ -332,6 +332,16 @@
      plans that use the expression index.
     </para>
    </tip>
+
+   <tip>
+    <para>
+     The autovacuum daemon does not issue <command>ANALYZE</> commands for
+     foreign tables, since it has no means of determining how often that
+     might be useful.  If your queries require statistics on foreign tables
+     for proper planning, it's a good idea to run manually-managed
+     <command>ANALYZE</> commands on those tables on a suitable schedule.
+    </para>
+   </tip>
   </sect2>
 
   <sect2 id="vacuum-for-visibility-map">