storage format version number for external modules

Right now the backend server has a catalog version number which reports
the version of the system tables and storage format.  It would be
helpful if pg_upgrade could access a storage format version number for
plugins like /contrib so it could check to see if the cluster can be
upgraded with the installed plugins.  (A catalog version number is not
necessary for plugins.)

--  Bruce Momjian  <bruce@momjian.us>        http://momjian.us EnterpriseDB
http://enterprisedb.com
 + It's impossible for everything to be true. +