RE: Enhance 'pg_createsubscriber' to retrieve databases automatically when no database is provided.

Dear Shubham,

> The attached patches contain the suggested changes.

Thanks for updating the patch. I reviewed only 0001 because they would be committed separately.
Few comments:

01.
```
+       For every non-template database on the source server, create one
+       subscription on the target server in the database with the same name.
```

It is quite confusing for me; We do not have to describe users that this command
checks databases of the source server. How about something like:
"Create one subscription per all non-template databases on the target server."

02.
```
+# run pg_createsubscriber with '--database' and '--all' without '--dry-run'
+# and verify the failure
+command_fails_like(
+	[
+		'pg_createsubscriber',
+		'--verbose',
+		'--pgdata' => $node_s->data_dir,
+		'--publisher-server' => $node_p->connstr($db1),
+		'--socketdir' => $node_s->host,
+		'--subscriber-port' => $node_s->port,
+		'--database' => $db1,
+		'--all',
+	],
+	qr/--database cannot be used with --all/,
+	'fail if --database is used with --all');
+
+# run pg_createsubscriber with '--publication' and '--all' and verify
+# the failure
+command_fails_like(
+	[
+		'pg_createsubscriber',
+		'--verbose',
+		'--dry-run',
+		'--pgdata' => $node_s->data_dir,
+		'--publisher-server' => $node_p->connstr($db1),
+		'--socketdir' => $node_s->host,
+		'--subscriber-port' => $node_s->port,
+		'--all',
+		'--publication' => 'pub1',
+	],
+	qr/--publication cannot be used with --all/,
+	'fail if --publication is used with --all');
```

You seemed to move most of validation checks to 0002. Can you tell me a reason
why they are remained?

03.
```
+# Verify that the required logical replication objects are created. The
+# expected count 3 refers to postgres, $db1 and $db2 databases.
```

Hmm, but since we did a dry-run, any objects are not created, right?

Best regards,
Hayato Kuroda
FUJITSU LIMITED