Raise a WARNING for max_slot_wal_keep_size in pg_createsubscriber.

Raise a WARNING for max_slot_wal_keep_size in pg_createsubscriber.

During the pg_createsubscriber execution, it is possible that the required
WAL is removed from the primary/publisher node due to
'max_slot_wal_keep_size'.

This patch raises a WARNING during the '--dry-run' mode if the
'max_slot_wal_keep_size' is set to a non-default value on the
primary/publisher node.

Author: Shubham Khanna <khannashubham1197@gmail.com>
Reviewed-by: Peter Smith <smithpb2250@gmail.com>
Reviewed-by: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Reviewed-by: Vignesh C <vignesh21@gmail.com>
Discussion: https://postgr.es/m/CAHv8Rj+deqsQXOMa7Tck8CBQUbsua=+4AuMVQ2=MPM0f-ZHbjA@mail.gmail.com