Automatically add update triggers for postgres updated_at columns

This may be a bit complex, but there is a "standardized" way of handling an updated_at column in postgres.

The idea is you create a function like this:

          CREATE FUNCTION update_updated_at_column() RETURNS trigger
            LANGUAGE plpgsql
            AS $$
          BEGIN
            NEW.updated_at = NOW();
            RETURN NEW;
          END;

This only has to be done once per database.

And then attach it to the column when it's created like this:

CREATE TRIGGER sometable_update_trigger BEFORE UPDATE ON sometable FOR EACH ROW EXECUTE PROCEDURE update_updated_at_column();

I'm doing this manually in migrations right now but it would be really cool if Phinx could do this automatically as part of the addTimestamps and addTimestampsWithTimezone calls.

Start at the addTimestamps and addTimestampsWithTimezone entry points and review how PostgreSQL migration statements are generated. Compare the requested function and trigger behavior with existing migration patterns; done means those calls can establish updated_at triggers without manual migration SQL.