Enhance 'pg_createsubscriber' to retrieve databases automatically when no database is provided.

Hi all,

I am writing to propose an enhancement to the pg_createsubscriber
utility that enables it to automatically fetch all non-template
databases from the publisher when no specific databases are specified
by the user. This was an open item from [1] that was planned for
future implementation. The attached patch has the changes for the
same.

[1] - https://www.postgresql.org/message-id/CAA4eK1%2BJpALAokLqxVsQKgo9iFrO-zChfvNXXJMkC8jUgYykBw%40mail.gmail.com

Thanks and regards,
Shubham Khanna.