Allow pgoutput to send logical decoding messages.

Allow pgoutput to send logical decoding messages.

The output plugin accepts a new parameter (messages) that controls if
logical decoding messages are written into the replication stream. It is
useful for those clients that use pgoutput as an output plugin and needs
to process messages that were written by pg_logical_emit_message().

Although logical streaming replication protocol supports logical
decoding messages now, logical replication does not use this feature yet.

Author: David Pirotte, Euler Taveira
Reviewed-by: Euler Taveira, Andres Freund, Ashutosh Bapat, Amit Kapila
Discussion: https://postgr.es/m/CADK3HHJ-+9SO7KuRLH=9Wa1rAo60Yreq1GFNkH_kd0=CdaWM+A@mail.gmail.com