The new command lists extended statistics objects. All past releases
with extended statistics are supported.

This is a simplified version of commit 891a1d0b, which had to be
reverted due to not considering pg_statistic_ext_data is not accessible
by regular users. Fields requiring access to this catalog were removed.
It's possible to add them, but it'll require changes to core.

Author: Tatsuro Yamada
Reviewed-by: Julien Rouhaud, Alvaro Herrera, Tomas Vondra, Noriyoshi Shinoda
Discussion: https://postgr.es/m/c027a541-5856-75a5-0868-341301e1624b%40nttcom.co.jp_1