NTP server support

[troglobit]

Description

This PR adds NTP server support using the ietf-ntp (RFC 9249) YANG model. The implementation supports three operational modes aligned with the documentation:

• Standalone mode: Uses only a local reference clock to serve time
• Server mode: Syncs from upstream NTP servers while serving clients
• Peer mode: Bidirectional synchronization between two NTP servers

Features Implemented

• Unicast server and peer configurations with composite key (address, type)
• Configurable poll intervals (minpoll/maxpoll), NTP version, per-source port
• Preference settings (prefer, iburst, burst with peer mode restrictions)
• makestep: Fast initial synchronization (1.0s threshold, 3 update limit by default)
• rtcsync: Hardware RTC synchronization enabled by default for time persistence across reboots
• Comprehensive operational state:
  • Server statistics (packets received/sent/dropped)
  • Associations (address, mode, stratum)
  • Clock state (stratum, offsets, delays, frequencies, sync state)
• Mutual exclusion with ietf-system NTP client enforced via YANG

Deviations & Limitations

Several advanced features from ietf-ntp have been deviated as not-supported:

• Authentication (keys and per-source auth)
• Access rules (currently allows all clients)
• Interface-specific configurations (broadcast/multicast modes)
• Source interface binding (chronyd limitation)

Testing

• server_mode_standalone: Standalone operation with local reference clock
• server_mode_server: Upstream sync while serving clients
• server_mode_peer: Bidirectional peer synchronization
• server_makestep: Configuration and operational state verification
• server_client: Verifies ietf-ntp (server) and ietf-system (client)

Checklist

Tick relevant boxes, this PR is-a or has-a:

• Bugfix
  • Regression tests
  • ChangeLog updates (for next release)
• Feature
  • YANG model change => revision updated?
  • Regression tests added?
  • ChangeLog updates (for next release)
  • Documentation added?
• Test changes
  • Checked in changed Readme.adoc (make test-spec)
  • Added new test to group Readme.adoc and yaml file
• Code style update (formatting, renaming)
• Refactoring (please detail in commit messages)
• Build related changes
• Documentation content changes
  • ChangeLog updated (for major changes)
• Other (please describe):