RE: Improve the error message for logical replication of regular column to generated column.

Dear Shubham,

Thanks for creating a patch! I checked yours and I have comments.

01.
```
+ StringInfoData gencolsattsbuf;
+ int generatedatts = 0;
+
+ initStringInfo(&gencolsattsbuf);
```

gencolsattsbuf is initialized at the beginning but won't be free'd.

But I prefer the Peter's suggestion - you can combine the reporting stuff to
logicalrep_report_missing_attrs and rename the function. This is clearer than
directly adding declarations and ereport() in logicalrep_rel_open().

02.

```
+ /*
+ * Check if the subscription table generated column has
+ * same name as a non-generated column in the
+ * corresponding publication table.
+ */
```

I don't think this comment is correct. The error can be reported even when
both publisher and subscriber has the generated column, right?
Also, I feel comments can be located atop "if".

03.
I feel if you combine the reporting stuff with logicalrep_report_missing_attrs, some
of changes are not needed anymore. You can just add comment in logicalrep_rel_open
and modify the message in logicalrep_report_missing_attrs.

[1]: https://www.postgresql.org/message-id/CAHut%2BPumbPEqk6v2XVjT7vKWKzQNBjMHXByWJ5%3DFmjEfk1v_pQ%40mail.gmail.com

Best regards,
Hayato Kuroda
FUJITSU LIMITED