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