feat: Add NTP (Network Time Protocol) monitoring support

[servak]

Summary

• Implement comprehensive NTP monitoring with time synchronization tracking
• Add support for both ntp://server[:port] and ntp:server[:port] formats
• Monitor response time and time offset from NTP servers with configurable thresholds
• Integrate seamlessly with existing ProbeManager and configuration system

Key Features

NTP Monitoring Capabilities

• Response Time Measurement: Track NTP query response times (similar to ping RTT)
• Time Offset Detection: Monitor time drift between local system and NTP server
• Configurable Thresholds: Set max_offset to alert on excessive time drift
• Multiple Server Support: Monitor multiple NTP servers simultaneously

Protocol Support

• New Format: ntp://time.google.com, ntp://pool.ntp.org:123
• Legacy Format: ntp:time.google.com, ntp:pool.ntp.org:123
• Default Configuration: Uses pool.ntp.org:123 with no offset limit

Configuration Example

prober:
  ntp-strict:
    probe: ntp
    ntp:
      server: "pool.ntp.org"
      port: 123
      max_offset: 100ms  # Alert if time drift > 100ms

Implementation Details

Files Added

• internal/prober/ntp.go: Core NTP prober implementation (400+ lines)
• internal/prober/ntp_test.go: Comprehensive test suite (200+ lines, 75+ test cases)

Files Modified

• internal/config/config.go: Add NTP default configuration
• internal/prober/config.go: Add NTP validation support
• internal/prober/manager.go: Integrate NTP prober creation
• internal/prober/validate_test.go: Add NTP validation tests

Test Coverage

• Configuration validation (server, port, max_offset ranges)
• Target parsing (format variations, error handling)
• NTP time conversion functions (RFC-compliant timestamp handling)
• Event registration and error handling
• Integration with ProbeManager

Test Results

Verified against real NTP servers with accurate results:

Response Time Monitoring

$ ./mping batch --count 2 ntp://time.google.com ntp://pool.ntp.org:123
┌────────────────────────┬──────┬──────┬──────┬────────┬───────┬───────┬───────┬───────┐
│ HOST                   │ SENT │ SUCC │ FAIL │ LOSS   │ LAST  │ AVG   │ BEST  │ WORST │
├────────────────────────┼──────┼──────┼──────┼────────┼───────┼───────┼───────┼───────┤
│ ntp://pool.ntp.org:123 │    2 │    2 │    0 │   0.0% │  14ms │  10ms │   7ms │  14ms │
│ ntp://time.google.com  │    2 │    2 │    0 │   0.0% │  51ms │  47ms │  43ms │  51ms │
└────────────────────────┴──────┴──────┴──────┴────────┴───────┴───────┴───────┴───────┘

Time Offset Detection

With strict 50ms offset limit:

$ ./mping batch --count 3 --config strict-ntp.yml ntp-strict://time.google.com
┌──────────────────────────────┬──────┬──────┬──────┬────────┬──────┬─────┬──────┬───────┬───────────┬───────────┬─────────────────────────────────────────────────┐
│ HOST                         │ SENT │ SUCC │ FAIL │ LOSS   │ LAST │ AVG │ BEST │ WORST │ LAST SUCC │ LAST FAIL │ FAIL REASON                                     │
├──────────────────────────────┼──────┼──────┼──────┼────────┼──────┼─────┼──────┼───────┼───────────┼───────────┼─────────────────────────────────────────────────┤
│ ntp-strict://time.google.com │    4 │    0 │    4 │ 100.0% │ -    │ -   │ -    │ -     │ -         │ 23:52:51  │ time offset too large: 107.444863ms (max: 50ms) │
└──────────────────────────────┴──────┴──────┴──────┴────────┴──────┴─────┴──────┴───────┴───────────┴───────────┴─────────────────────────────────────────────────┘

Accuracy Verification

Compared against system sntp tool:

$ sntp time.google.com
+0.105244 +/- 0.042827 time.google.com 216.239.35.4

# mping detected: 107.444863ms offset
# sntp detected:  105.244ms offset  
# Difference: <3ms (excellent accuracy)

Test Plan

• Verify NTP packet creation and parsing follows RFC standards
• Test response time measurement accuracy
• Test time offset calculation against system tools
• Validate configuration parsing and validation
• Test integration with existing prober architecture
• Verify error handling for unreachable servers
• Test threshold-based alerting functionality
• Confirm backward compatibility with existing features

Closes #38