Add functions to do timestamptz arithmetic in a non-default timezone.

Add versions of timestamptz + interval, timestamptz - interval, and
generate_series(timestamptz, ...) in which a timezone can be specified
explicitly instead of defaulting to the TimeZone GUC setting.

The new functions for the first two are named date_add and
date_subtract.  This might seem too generic, but we could use
overloading to add additional variants if that seems useful.

Along the way, improve the docs' pretty inadequate explanation
of how timestamptz +- interval works.

Przemysław Sztoch and Gurjeet Singh; cosmetic changes and most of
the docs work by me

Discussion: https://postgr.es/m/01a84551-48dd-1359-bf7e-f6b0203a6bd0@sztoch.pl

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