Give a more specific error message than "xyz is not a table".

Also document in CREATE PUBLICATION which kinds of relations are not
supported.

based on patch by Amit Langote <Langote_Amit_f8@lab.ntt.co.jp>

Author: Amit Langote <Langote_Amit_f8@lab.ntt.co.jp>

Add/correct documentation and add some tests related to how access
control around adding tables to publications works.

- Add PUBLICATION catalogs and DDL
- Add SUBSCRIPTION catalog and DDL
- Define logical replication protocol and output plugin
- Add logical replication workers

From: Petr Jelinek <petr@2ndquadrant.com>
Reviewed-by: Steve Singer <steve@ssinger.info>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Erik Rijkers <er@xs4all.nl>
Reviewed-by: Peter Eisentraut <peter.eisentraut@2ndquadrant.com>