Show opclass and opfamily related information in psql

Show opclass and opfamily related information in psql

This commit provides psql commands for listing operator classes, operator
families and its contents in psql.  New commands will be useful for exploring
capabilities of both builtin opclasses/opfamilies as well as
opclasses/opfamilies defined in extensions.

Discussion: https://postgr.es/m/1529675324.14193.5.camel%40postgrespro.ru
Author: Sergey Cherkashin, Nikita Glukhov, Alexander Korotkov
Reviewed-by: Michael Paquier, Alvaro Herrera, Arthur Zakirov
Reviewed-by: Kyotaro Horiguchi, Andres Freund