Allow for use of cheap USB GPS modules (eg VK-172)

I've been playing around with a cheap USB GPS module, the $9.99 [WWZMDiB VK-172 USB GPS Dongle Receiver](https://amzn.to/3EC1Zbb).

It may be nice to find a way to hack it's PPS output into a reference point on the Pi or on a NIC, for a low-cost entry-level option (to get someone's feet wet with a ~30-60ns accurate Pi Time server for much less than $100 all-in).

I wrote a blog post about the module, testing it on my Mac at home (I was pleasantly surprised): [Trying out a cheap USB VK-172 GPS dongle on a Mac](https://www.jeffgeerling.com/blog/2025/trying-out-cheap-usb-vk-172-gps-dongle-on-mac).

Start by reviewing the repository’s existing GPS/PPS and Raspberry Pi time-server setup, then compare it with the VK-172’s PPS output described in the linked blog post. Define the supported wiring and configuration, and verify that the module can provide the intended reference; the issue is done when a reproducible setup is documented and tested.

Written by the indexing model from the issue text.