… interfaces (#276)

* feat(exporters): add UDP transport abstraction

Add udp_transport.h with abstract interface and implementations:
- udp_transport: abstract base class for UDP communication
- stub_udp_transport: testing implementation with simulated sends
- common_udp_transport: uses common_system IUdpClient interface
- network_udp_transport: uses network_system UDP client

Includes factory functions for creating transports with automatic
backend selection based on available compile-time flags.

Part of #273

* feat(exporters): integrate UDP transport into statsd_exporter

Update statsd_exporter to use udp_transport abstraction:
- Add udp_transport member with factory-based creation
- Support custom transport injection for testing
- Implement start/stop lifecycle with connection management
- Replace stub send_udp_batch with real transport calls
- Add transport statistics to get_stats()

The exporter now sends real UDP packets when network backend
is available, with graceful fallback to stub for testing.

Part of #273, resolves #274

* feat(exporters): add gRPC transport and update OTLP exporter

Add grpc_transport.h with abstract interface and implementations:
- grpc_transport: abstract base class for gRPC communication
- stub_grpc_transport: testing implementation with simulated calls
- Placeholder for network_grpc_transport when gRPC lib available

Update otlp_metrics_exporter to use transport abstractions:
- Add http_transport and grpc_transport members
- Implement send_via_http for OTLP/HTTP JSON and Protobuf
- Implement send_via_grpc for OTLP/gRPC protocol
- Add start/stop lifecycle management
- Include transport statistics in get_stats()
- Add basic OTLP JSON serialization

Part of #273, resolves #275

* test(exporters): add UDP and gRPC transport tests

Add comprehensive tests for transport layer:
- UdpTransportTest: stub transport functionality, failure simulation,
  statistics tracking and reset
- GrpcTransportTest: stub transport functionality, custom response
  handlers, failure simulation
- Integration tests for statsd_exporter with custom UDP transport
- Integration tests for otlp_metrics_exporter with custom HTTP/gRPC
  transports

Part of #273

* build(cmake): add transport interface detection

Add CMake configuration to detect common_system transport interfaces:
- Check for transport.h in multiple locations
- Define MONITORING_HAS_COMMON_TRANSPORT_INTERFACES when found
- Enable IHttpClient and IUdpClient integration automatically

This allows udp_transport.h and grpc_transport.h to use common_system
transport interfaces when available.

Part of #273

* docs: update CHANGELOG with transport implementations

Document new transport features in both English and Korean:
- UDP and gRPC transport abstractions (#273)
- StatsD exporter with real UDP transport (#274)
- OTLP exporter with HTTP/gRPC transport (#275)
- CMake transport interface detection

Part of #273