ALTER SUBSCRIPTION ... ADD/DROP PUBLICATION

At present, if we want to update publications in a subscription, we
can use SET PUBLICATION.  However, it requires supplying all
publications that exists and the new publications.  If we want to add
new publications, it's inconvenient.  The new syntax only supplies the
new publications.  When the refresh is true, it only refreshes the new
publications.

Author: Japin Li <japinli@hotmail.com>
Author: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/MEYP282MB166939D0D6C480B7FBE7EFFBB6BC0@MEYP282MB1669.AUSP282.PROD.OUTLOOK.COM
