RE: Pgoutput not capturing the generated columns

Dear Vignesh,

Thanks for rebasing the patch! Before reviewing deeply, I want to confirm the specification.
I understood like below based on the documentation.

- If publish_generated_columns is false, the publication won't replicate generated columns
- If publish_generated_columns is true, the behavior on the subscriber depends on the table column:
- If it is a generated column even on the subscriber, it causes an ERROR.
- If it is a regular column, the generated value is replicated.
- If the column is missing, it causes an ERROR.

However, below test implies that generated columns can be replicated even if
publish_generated_columns is false. Not sure...

```
# Verify that incremental replication of generated columns occurs
# when they are included in the column list, regardless of the
# publish_generated_columns option.
$result =
$node_subscriber->safe_psql('postgres', "SELECT * FROM tab3 ORDER BY a");
is( $result, qq(|2
|4
|6
|8),
'tab3 incremental replication, when publish_generated_columns=false');
```

Also, I've tested the case both pub and sub had the generated column, but the ERROR was strange for me.

```
test_pub=# CREATE TABLE gencoltable (a int PRIMARY KEY, b int GENERATED ALWAYS AS (a * 2) STORED);
test_pub=# CREATE PUBLICATION pub FOR TABLE gencoltable(a, b) WITH (publish_generated_columns = true);
test_pub=# INSERT INTO gencoltable (a) VALUES (generate_series(1, 10));

test_sub=# CREATE TABLE gencoltable (a int PRIMARY KEY, b int GENERATED ALWAYS AS (a * 2) STORED);
test_sub=# CREATE SUBSCRIPTION sub CONNECTION ... PUBLICATION pub;

-> ERROR: logical replication target relation "public.gencoltable" is missing replicated column: "b"
```

The attribute existed on the sub but it was reported as missing column. I think
we should somehow report like "generated column on publisher is replicated the
generated column on the subscriber".

Best regards,
Hayato Kuroda
FUJITSU LIMITED