psql \d: Display table where trigger is defined, if inherited

psql \d: Display table where trigger is defined, if inherited

It's important to know that a trigger is cloned from a parent table,
because of the behavior that the trigger is dropped on detach.  Make
psql's \d display it.

We'd like to backpatch, but lack of the pg_trigger.tgparentid column
makes it more difficult.  Punt for now.  If somebody wants to volunteer
an implementation that reads pg_depend on older versions, that can
probably be backpatched.

Authors: Justin Pryzby, Amit Langote, Álvaro Herrera
Discussion: https://postgr.es/m/20200419002206.GM26953@telsasoft.com