Add PPS source support

[michielp1807]

Builds on #1768 to be able to use Pulse Per Second devices in ntpd-rs using the pps-time crate, closing #1633.

Example config:

[[source]]
mode = "sock"
path = "/run/chrony.ttyAMA0.sock"
measurement_noise_estimate = 0.001

[[source]]
mode = "pps"
path = "/dev/pps0"
measurement_noise_estimate = 0.0001

TODO:

• Add documentation for using PPS sources in ntpd-rs
• During testing I noticed it would run into this debug assert when using GPSD. It would probably be good to do some more testing 🙂