postgres_fdw: Disable batch insert when BEFORE ROW INSERT triggers exist.

Previously, we allowed this, but such triggers might query the table to
insert into and act differently if the tuples that have already been
processed and prepared for insertion are not there, so disable it in
such cases.

Back-patch to v14 where batch insert was added.

Discussion: https://postgr.es/m/CAPmGK16_uPqsmgK0-LpLSUk54_BoK13bPrhxhfjSoSTVz414hA%40mail.gmail.com

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -10008,6 +10008,34 @@ SELECT COUNT(*) FROM ftable;
      2
 (1 row)
 
+-- Disable batch inserting into foreign tables with BEFORE ROW INSERT triggers
+-- even if the batch_size option is enabled.
+ALTER FOREIGN TABLE ftable OPTIONS ( SET batch_size '10' );
+CREATE TRIGGER trig_row_before BEFORE INSERT ON ftable
+FOR EACH ROW EXECUTE PROCEDURE trigger_data(23,'skidoo');
+EXPLAIN (VERBOSE, COSTS OFF) INSERT INTO ftable VALUES (3), (4);
+                         QUERY PLAN                          
+-------------------------------------------------------------
+ Insert on public.ftable
+   Remote SQL: INSERT INTO public.batch_table(x) VALUES ($1)
+   Batch Size: 1
+   ->  Values Scan on "*VALUES*"
+         Output: "*VALUES*".column1
+(5 rows)
+
+INSERT INTO ftable VALUES (3), (4);
+NOTICE:  trig_row_before(23, skidoo) BEFORE ROW INSERT ON ftable
+NOTICE:  NEW: (3)
+NOTICE:  trig_row_before(23, skidoo) BEFORE ROW INSERT ON ftable
+NOTICE:  NEW: (4)
+SELECT COUNT(*) FROM ftable;
+ count 
+-------
+     4
+(1 row)
+
+-- Clean up
+DROP TRIGGER trig_row_before ON ftable;
 DROP FOREIGN TABLE ftable;
 DROP TABLE batch_table;
 -- Use partitioning