Allow publishing partition changes via ancestors

Allow publishing partition changes via ancestors

To control whether partition changes are replicated using their own
identity and schema or an ancestor's, add a new parameter that can be
set per publication named 'publish_via_partition_root'.

This allows replicating a partitioned table into a different partition
structure on the subscriber.

Author: Amit Langote 
Reviewed-by: Rafia Sabih 
Reviewed-by: Peter Eisentraut 
Reviewed-by: Petr Jelinek 
Discussion: https://www.postgresql.org/message-id/flat/CA+HiwqH=Y85vRK3mOdjEkqFK+E=ST=eQiHdpj43L=_eJMOOznQ@mail.gmail.com