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 = 1e-6

[[source]]
mode = "pps"
path = "/dev/pps0"
measurement_noise_estimate = 1e-14

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 🙂
• For some reason (likely precision problems?) things go wrong with low noise estimates (e.g. 1e-14), it does seem to work with a 1e-8 noise estimate -> could this be due to rounding errors with NtpDuration, where to_seconds / from_seconds are not exact?
• Update ntp-ctl status to also show PPS/Sock sources

[codecov]

Codecov Report

Attention: Patch coverage is 81.38007% with 170 lines in your changes missing coverage. Please review.

Project coverage is 81.49%. Comparing base (419832f) to head (101704c).

Files with missing lines	Patch %	Lines
ntpd/src/daemon/pps_source.rs	0.00%	90 Missing ⚠️
ntpd/src/daemon/system.rs	0.00%	24 Missing ⚠️
ntpd/src/daemon/spawn/pps.rs	80.00%	15 Missing ⚠️
ntp-proto/src/system.rs	23.52%	13 Missing ⚠️
ntp-proto/src/algorithm/kalman/source.rs	97.94%	11 Missing ⚠️
ntpd/src/daemon/config/ntp_source.rs	45.45%	6 Missing ⚠️
ntpd/src/force_sync/algorithm.rs	0.00%	6 Missing ⚠️
ntpd/src/daemon/config/mod.rs	0.00%	2 Missing ⚠️
ntpd/src/daemon/spawn/mod.rs	50.00%	2 Missing ⚠️
ntpd/src/force_sync/mod.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1776      +/-   ##
==========================================
- Coverage   81.55%   81.49%   -0.06%     
==========================================
  Files          66       68       +2     
  Lines       20690    21485     +795     
==========================================
+ Hits        16874    17510     +636     
- Misses       3816     3975     +159     

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