Prepare to support non-tables in publications

Prepare to support non-tables in publications

This by itself doesn't change any functionality but prepares the way
for having relations other than base tables in publications.

Make arrangements for COPY handling the initial table sync.  For
non-tables we have to use COPY (SELECT ...) instead of directly
copying from the table, but then we have to take care to omit
generated columns from the column list.

Also, remove a hardcoded reference to relkind = 'r' and rely on the
publisher to send only what it can actually publish, which will be
correct even in future cross-version scenarios.

Reviewed-by: Amit Langote <amitlangote09@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CA+HiwqH=Y85vRK3mOdjEkqFK+E=ST=eQiHdpj43L=_eJMOOznQ@mail.gmail.com