amcheck: Add gin_index_check() to verify GIN index

Adds a new function, validating two kinds of invariants on a GIN index:

- parent-child consistency: Paths in a GIN graph have to contain
  consistent keys. Tuples on parent pages consistently include tuples
  from child pages; parent tuples do not require any adjustments.

- balanced-tree / graph: Each internal page has at least one downlink,
  and can reference either only leaf pages or only internal pages.

The GIN verification is based on work by Grigory Kryachko, reworked by
Heikki Linnakangas and with various improvements by Andrey Borodin.
Investigation and fixes for multiple bugs by Kirill Reshke.

Author: Grigory Kryachko <GSKryachko@gmail.com>
Author: Heikki Linnakangas <hlinnaka@iki.fi>
Author: Andrey Borodin <amborodin@acm.org>
Reviewed-By: José Villanova <jose.arthur@gmail.com>
Reviewed-By: Aleksander Alekseev <aleksander@timescale.com>
Reviewed-By: Nikolay Samokhvalov <samokhvalov@gmail.com>
Reviewed-By: Andres Freund <andres@anarazel.de>
Reviewed-By: Tomas Vondra <tomas.vondra@enterprisedb.com>
Reviewed-By: Kirill Reshke <reshkekirill@gmail.com>
Reviewed-By: Mark Dilger <mark.dilger@enterprisedb.com>
Reviewed-By: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/45AC9B0A-2B45-40EE-B08F-BDCF5739D1E1%40yandex-team.ru

doc/src/sgml/amcheck.sgml

@@ -188,6 +188,26 @@ ORDER BY c.relpages DESC LIMIT 10;
      </para>
     </listitem>
    </varlistentry>
+
+   <varlistentry>
+    <term>
+     <function>gin_index_check(index regclass) returns void</function>
+     <indexterm>
+      <primary>gin_index_check</primary>
+     </indexterm>
+    </term>
+
+    <listitem>
+     <para>
+      <function>gin_index_check</function> tests that its target GIN index
+      has consistent parent-child tuples relations (no parent tuples
+      require tuple adjustement) and page graph respects balanced-tree
+      invariants (internal pages reference only leaf page or only internal
+      pages).
+     </para>
+    </listitem>
+   </varlistentry>
+
   </variablelist>
   <tip>
    <para>