sql: include hidden columns in triggers

[DrewKimball]

Now that it is possible to assign to an element of a composite-typed PL/pgSQL variable (#132628), we should make hidden columns visible to triggers, in addition to visible columns.

Jira issue: CRDB-43543

[DrewKimball]

Note that until this is fixed, triggers will encounter an assertion error upon execution on a table with the implicit rowid >column. For example, the following optimizer test:

exec-ddl
CREATE TABLE t (a INT);
----

exec-ddl CREATE FUNCTION f() RETURNS TRIGGER LANGUAGE PLpgSQL AS $$ BEGIN RETURN NEW; END $$;

exec-ddl CREATE TRIGGER tr BEFORE INSERT ON t FOR EACH ROW EXECUTE FUNCTION f();

build INSERT INTO t VALUES (1);

>encounters: "unexpected number of visible columns"

It turns out the assertion failure was an artifact of some missing handling for hidden columns in opt tests. However, we can still keep this issue open to track adding support for hidden columns to triggers.