telemetry/geoprobe: add probe measurement and offset delivery

[ben-dz]

Resolves: #2899

Summary of Changes

• Add coordinator module for lifecycle management of geoprobe measurement operations
• Add pinger module for TWAMP-based RTT measurements to child probes with retry support
• Add publisher module for generating and delivering signed LocationOffset structures via UDP
• Add security hardening including IP validation (reject private/loopback/link-local/reserved addresses), concurrency limits (100 max concurrent operations), reference chain validation, and DNS resolution timeout
• Integrate geoprobe functionality into telemetry collector via new goroutine when --additional-child-probes CLI flag is provided
• Add configuration validation for geoprobe dependencies (serviceability client and RPC client required when geoprobe enabled)
• Extend main.go with --additional-child-probes and --management-namespace CLI flags
  This implements RFC16 Phase 1, item 1 (Telemetry agent extensions) and enables DZDs to measure RTT to child geoProbes, generate signed offset structures, and deliver them for geolocation verification.

Testing Verification

• Comprehensive unit tests added for all new modules (coordinator, pinger, publisher, address validation)
• Test coverage includes security validation, concurrency handling, error cases, and reference chain verification
• Configuration validation tests verify proper dependency checks
• All tests passing locally