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

src/bin/psql/command.c

@@ -928,6 +928,9 @@ exec_command_d(PsqlScanState scan_state, bool active_branch, const char *cmd)
 				else
 					success = listExtensions(pattern);
 				break;
+			case 'X':			/* Extended Statistics */
+				success = listExtendedStats(pattern);
+				break;
 			case 'y':			/* Event Triggers */
 				success = listEventTriggers(pattern, show_verbose);
 				break;