"nts-pool" mode for ntpd client

[squell]

This adds a "nts-pool" spawner to the ntpd-rs client. The spawner is closely based on the existing ones for plain NTS and NTP-pool.

This can be reviewed commit-by-commit.

[codecov]

Codecov Report

Attention: 136 lines in your changes are missing coverage. Please review.

Comparison is base (a6fbc9f) 82.85% compared to head (706bd85) 82.29%.

Files	Patch %	Lines
ntpd/src/daemon/spawn/nts_pool.rs	0.00%	115 Missing ⚠️
ntpd/src/daemon/keyexchange.rs	40.00%	9 Missing ⚠️
ntpd/src/daemon/system.rs	0.00%	7 Missing ⚠️
ntp-proto/src/nts_pool_ke.rs	0.00%	2 Missing ⚠️
ntpd/src/daemon/config/peer.rs	85.71%	2 Missing ⚠️
ntpd/src/daemon/config/mod.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1241      +/-   ##
==========================================
- Coverage   82.85%   82.29%   -0.56%     
==========================================
  Files          65       66       +1     
  Lines       18847    19006     +159     
==========================================
+ Hits        15615    15641      +26     
- Misses       3232     3365     +133     

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