ext/pgsql: adding pg_service() alongside other connection infos.

returns the ongoing name of the service, if there is.
available since postgres 18

close GH-18198

ext/pgsql/pgsql.stub.php

@@ -508,6 +508,9 @@ namespace {
      */
     function pg_jit(?PgSql\Connection $connection = null): array {}
 
+#ifdef HAVE_PG_SERVICE
+    function pg_service(?PgSql\Connection $connection = null): string {}
+#endif
     /**
      * @param PgSql\Connection|string $connection
      * @refcount 1