list of extended statistics on psql

Hi!

I created a POC patch that allows showing a list of extended statistics by
"\dz" command on psql. I believe this feature helps DBA and users who
would like to know all extended statistics easily. :-D

I have not a strong opinion to assign "\dz". I prefer "\dx" or "\de*"
than "\dz" but they were already assigned. Therefore I used "\dz"
instead of them.

Please find the attached patch.
Any comments are welcome!

For Example:
=======================
CREATE TABLE t1 (a INT, b INT);
CREATE STATISTICS stts1 (dependencies) ON a, b FROM t1;
CREATE STATISTICS stts2 (dependencies, ndistinct) ON a, b FROM t1;
CREATE STATISTICS stts3 (dependencies, ndistinct, mcv) ON a, b FROM t1;
ANALYZE t1;

CREATE TABLE t2 (a INT, b INT, c INT);
CREATE STATISTICS stts4 ON b, c FROM t2;
ANALYZE t2;

postgres=# \dz
List of extended statistics
Schema | Table | Name | Columns | Ndistinct | Dependencies | MCV
--------+-------+-------+---------+-----------+--------------+-----
public | t1 | stts1 | a, b | f | t | f
public | t1 | stts2 | a, b | t | t | f
public | t1 | stts3 | a, b | t | t | t
public | t2 | stts4 | b, c | t | t | t
(4 rows)

postgres=# \?
...
\dy [PATTERN] list event triggers
\dz [PATTERN] list extended statistics
\l[+] [PATTERN] list databases
...
=======================

For now, I haven't written a document and regression test for that.
I'll create it later.

Thanks,
Tatsuro Yamada