Add stratnum GiST support function

This is support function 12 for the GiST AM and translates
"well-known" RT*StrategyNumber values into whatever strategy number is
used by the opclass (since no particular numbers are actually
required).  We will use this to support temporal PRIMARY
KEY/UNIQUE/FOREIGN KEY/FOR PORTION OF functionality.

This commit adds two implementations, one for internal GiST opclasses
(just an identity function) and another for btree_gist opclasses.  It
updates btree_gist from 1.7 to 1.8, adding the support function for
all its opclasses.

(previously committed as 6db4598fcb, reverted by 8aee330af55; this is
essentially unchanged from those)

Author: Paul A. Jungwirth <pj@illuminatedcomputing.com>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Reviewed-by: jian he <jian.universality@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CA+renyUApHgSZF9-nd-a0+OPGharLQLO=mDHcY4_qQ0+noCUVg@mail.gmail.com

contrib/btree_gist/meson.build

@@ -50,6 +50,7 @@ install_data(
   'btree_gist--1.4--1.5.sql',
   'btree_gist--1.5--1.6.sql',
   'btree_gist--1.6--1.7.sql',
+  'btree_gist--1.7--1.8.sql',
   kwargs: contrib_data_args,
 )
 
@@ -89,6 +90,7 @@ tests += {
       'enum',
       'bool',
       'partitions',
+      'stratnum',
     ],
   },
 }