pgsql: Add GUC for temporarily disabling event triggers

Add GUC for temporarily disabling event triggers

In order to troubleshoot misbehaving or buggy event triggers, the
documented advice is to enter single-user mode.  In an attempt to
reduce the number of situations where single-user mode is required
(or even recommended) for non-extraordinary maintenance, this GUC
allows to temporarily suspend event triggers.

This was originally extracted from a larger patchset which aimed
at supporting event triggers on login events.

Reviewed-by: Ted Yu <yuzhihong@gmail.com>
Reviewed-by: Mikhail Gribkov <youzhick@gmail.com>
Reviewed-by: Justin Pryzby <pryzby@telsasoft.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Discussion: https://postgr.es/m/9140106E-F9BF-4D85-8FC8-F2D3C094A6D9@yesql.se
Discussion: https://postgr.es/m/0d46d29f-4558-3af9-9c85-7774e14a7709@postgrespro.ru

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/7750fefdb2b81e0ee09c523d0a21cbd37edaf4b2

Modified Files
--------------
doc/src/sgml/config.sgml                      | 19 +++++++++++++++++++
doc/src/sgml/ref/create_event_trigger.sgml    |  9 ++++++---
src/backend/commands/event_trigger.c          | 20 +++++++++++++-------
src/backend/utils/misc/guc_tables.c           | 11 +++++++++++
src/backend/utils/misc/postgresql.conf.sample |  1 +
src/include/commands/event_trigger.h          |  2 ++
src/test/regress/expected/event_trigger.out   | 22 ++++++++++++++++++++++
src/test/regress/sql/event_trigger.sql        | 24 ++++++++++++++++++++++++
8 files changed, 98 insertions(+), 10 deletions(-)