GPS time conversion

There is a strong need for GPS time conversion in many science applications. Unfortunately none of the existing python date/time libraries support GPS time conversion. The only library that does on a limited basis is [astropy](https://www.astropy.org). I am the upstream maintainer of [gpstime](https://pypi.org/project/gpstime/), which is an attempt to fill that gap. I (and likely much of the scientific community) would be extremely thrilled if this functionality was just folded into arrow.

## Feature Request

GPS time conversion support.

Start by reviewing Arrow’s existing date and time conversion APIs, then compare the requested behavior with the linked gpstime project and the limited support in astropy. Define the supported GPS conversion scope and expected results before implementation; done means GPS time conversion is available in Arrow and its behavior is verified against the chosen reference.

Written by the indexing model from the issue text.