Add sock source for GPSD socket

[michielp1807]

This PR builds on #1673 to add a sock time source for GPSD sockets.

The sock source can for example be configured as follows in the config file:

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

The NTP daemon will create the socket, which means that if you (re-)start GPSD, GPSD will send time data to us.

[codecov]

Codecov Report

Attention: Patch coverage is 73.46939% with 182 lines in your changes missing coverage. Please review.

Project coverage is 81.44%. Comparing base (dfdcb5b) to head (bb9c4fd).
Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
ntpd/src/daemon/system.rs	0.00%	51 Missing ⚠️
ntpd/src/daemon/sock_source.rs	83.84%	37 Missing ⚠️
ntpd/src/force_sync/algorithm.rs	0.00%	32 Missing ⚠️
ntp-proto/src/system.rs	55.81%	19 Missing ⚠️
ntpd/src/daemon/spawn/sock.rs	79.45%	15 Missing ⚠️
ntp-proto/src/source.rs	70.96%	9 Missing ⚠️
ntpd/src/daemon/spawn/mod.rs	57.89%	8 Missing ⚠️
ntp-proto/src/algorithm/kalman/source.rs	97.36%	4 Missing ⚠️
ntpd/src/daemon/config/mod.rs	0.00%	2 Missing ⚠️
ntpd/src/daemon/config/ntp_source.rs	71.42%	2 Missing ⚠️
... and 3 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1674      +/-   ##
==========================================
- Coverage   81.54%   81.44%   -0.11%     
==========================================
  Files          64       66       +2     
  Lines       20090    20496     +406     
==========================================
+ Hits        16382    16692     +310     
- Misses       3708     3804      +96     

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