[C++] Kernel to convert timestamp with timezone to another timezone (metadata-only change)

Related to ARROW-13033, but assuming you already have a tz-aware "timestamp with timezone" type, and you want to change it to have another timezone.

In practice, this is a trivial, metadata-only change since in both cases the actual values are stored in UTC, and so it's only changing the tz parametrization of the timestamp type. But, for convenience, I think it would still be useful to have it as an actual kernel to easily integrate this operation in this framework.

References: tz_convert in pandas, or with_tz in R's lubridate package,withZone in Joda or withZoneSameInstant in Java