Fix confusion about event trigger vs. plain function in plpgsql.

The function hash table keys made by compute_function_hashkey() failed
to distinguish event-trigger call context from regular call context.
This meant that once we'd successfully made a hash entry for an event
trigger (either by validation, or by normal use as an event trigger),
an attempt to call the trigger function as a plain function would
find this hash entry and thereby bypass the you-can't-do-that check in
do_compile().  Thus we'd attempt to execute the function, leading to
strange errors or even crashes, depending on function contents and
server version.

To fix, add an isEventTrigger field to PLpgSQL_func_hashkey,
paralleling the longstanding infrastructure for regular triggers.
This fits into what had been pad space, so there's no risk of an ABI
break, even assuming that any third-party code is looking at these
hash keys.  (I considered replacing isTrigger with a PLpgSQL_trigtype
enum field, but felt that that carried some API/ABI risk.  Maybe we
should change it in HEAD though.)

Per bug #16266 from Alexander Lakhin.  This has been broken since
event triggers were invented, so back-patch to all supported branches.

Discussion: https://postgr.es/m/16266-fcd7f838e97ba5d4@postgresql.org

src/test/regress/expected/event_trigger.out

@@ -9,6 +9,10 @@ BEGIN
     RAISE NOTICE 'test_event_trigger: % %', tg_event, tg_tag;
 END
 $$ language plpgsql;
+-- should fail, can't call it as a plain function
+SELECT test_event_trigger();
+ERROR:  trigger functions can only be called as triggers
+CONTEXT:  compilation of PL/pgSQL function "test_event_trigger" near line 1
 -- should fail, event triggers cannot have declared arguments
 create function test_event_trigger_arg(name text)
 returns event_trigger as $$ BEGIN RETURN 1; END $$ language plpgsql;

src/test/regress/sql/event_trigger.sql

@@ -10,6 +10,9 @@ BEGIN
 END
 $$ language plpgsql;
 
+-- should fail, can't call it as a plain function
+SELECT test_event_trigger();
+
 -- should fail, event triggers cannot have declared arguments
 create function test_event_trigger_arg(name text)
 returns event_trigger as $$ BEGIN RETURN 1; END $$ language plpgsql;