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