Support event trigger for Neon users (#10624)

## Problem

https://github.com/neondatabase/neon/issues/7570

Even triggers are supported only for superusers.

## Summary of changes

Temporary switch to superuser when even trigger is created and disable
execution of user's even triggers under superuser.

---------

Co-authored-by: Dimitri Fontaine <dim@tapoueh.org>
Co-authored-by: Konstantin Knizhnik <knizhnik@neon.tech>

test_runner/sql_regress/expected/neon-event-triggers.out

@@ -0,0 +1,90 @@
+create or replace function admin_proc()
+    returns event_trigger
+    language plpgsql as
+$$
+begin
+    raise notice 'admin event trigger is executed for %', current_user;
+end;
+$$;
+create role neon_superuser;
+create role neon_admin login inherit createrole createdb in role neon_superuser;
+grant create on schema public to neon_admin;
+create database neondb with owner neon_admin;
+grant all privileges on database neondb to neon_superuser;
+create role neon_user;
+grant create on schema public to neon_user;
+create event trigger on_ddl1 on ddl_command_end
+execute procedure admin_proc();
+set role neon_user;
+-- check that non-privileged user can not change neon.event_triggers
+set neon.event_triggers to false;
+ERROR:  permission denied to set neon.event_triggers
+DETAIL:  Only "neon_superuser" is allowed to set the GUC
+-- Non-privileged neon user should not be able to create event trigers
+create event trigger on_ddl2 on ddl_command_end
+execute procedure admin_proc();
+ERROR:  permission denied to create event trigger "on_ddl2"
+HINT:  Must be superuser to create an event trigger.
+set role neon_admin;
+-- neon_superuser should be able to create event trigers
+create or replace function neon_proc()
+    returns event_trigger
+    language plpgsql as
+$$
+begin
+    raise notice 'neon event trigger is executed for %', current_user;
+end;
+$$;
+NOTICE:  admin event trigger is executed for neon_admin
+create event trigger on_ddl2 on ddl_command_end
+execute procedure neon_proc();
+\c neondb neon_admin
+create or replace function neondb_proc()
+    returns event_trigger
+    language plpgsql as
+$$
+begin
+    raise notice 'neondb event trigger is executed for %', current_user;
+end;
+$$;
+create or replace function neondb_secdef_proc()
+    returns event_trigger
+    language plpgsql
+    SECURITY DEFINER
+as
+$$
+begin
+    raise notice 'neondb secdef event trigger is executed for %', current_user;
+end;
+$$;
+-- neon_admin (neon_superuser member) should be able to create event triggers
+create event trigger on_ddl3 on ddl_command_end
+execute procedure neondb_proc();
+create event trigger on_ddl4 on ddl_command_end
+execute procedure neondb_secdef_proc();
+-- Check that event trigger is fired for neon_admin
+create table t1(x integer);
+NOTICE:  neondb event trigger is executed for neon_admin
+NOTICE:  neondb secdef event trigger is executed for neon_admin
+-- Check that event trigger can be skipped
+set neon.event_triggers to false;
+create table t2(x integer);
+WARNING:  Skipping Event Trigger: neon.event_triggers is false
+WARNING:  Skipping Event Trigger: neon.event_triggers is false
+\c regression cloud_admin
+-- Check that event triggers are not fired for superuser
+create table t3(x integer);
+NOTICE:  admin event trigger is executed for cloud_admin
+WARNING:  Skipping Event Trigger
+DETAIL:  Event Trigger function "neon_proc" is owned by non-superuser role "neon_admin", and current_user "cloud_admin" is superuser
+\c neondb cloud_admin
+-- Check that user-defined event triggers are not fired for superuser
+create table t4(x integer);
+WARNING:  Skipping Event Trigger
+DETAIL:  Event Trigger function "neondb_proc" is owned by non-superuser role "neon_admin", and current_user "cloud_admin" is superuser
+WARNING:  Skipping Event Trigger
+DETAIL:  Event Trigger function "neondb_secdef_proc" is owned by non-superuser role "neon_admin", and current_user "cloud_admin" is superuser
+\c neondb neon_admin
+-- Check that neon_admin can drop event triggers
+drop event trigger on_ddl3;
+drop event trigger on_ddl4;