Enable formatting of valid_to & valid_from in SCD T2 models

[mattiasthalen]

I would like the option to format the valid_to & valid_from fields.
Currently they are of the type TIMESTAMP and %Y-%m-%d %H:%M:%S.
I would like to have it as TIMESTAMP WITH TIME ZONE and %Y-%m-%d %H:%M:%S%z.

In other words, I want to be explicit about the time zone, in order to be consistent with other timestamps in my project.