pgsql: psql \dX: list extended statistics objects

psql \dX: list extended statistics objects

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

This is a simplified version of commit 891a1d0bca, 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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/ad600bba0422dde4b73fbd61049ff2a3847b068a

Modified Files
--------------
doc/src/sgml/ref/psql-ref.sgml          |  21 ++++++
src/bin/psql/command.c                  |   3 +
src/bin/psql/describe.c                 |  83 +++++++++++++++++++++++
src/bin/psql/describe.h                 |   3 +
src/bin/psql/help.c                     |   1 +
src/bin/psql/tab-complete.c             |   4 +-
src/test/regress/expected/stats_ext.out | 116 ++++++++++++++++++++++++++++++++
src/test/regress/sql/stats_ext.sql      |  37 ++++++++++
8 files changed, 267 insertions(+), 1 deletion(-)