feat(logs): add support for otlp logs export

[mabdinur]

What does this PR do?

Adds OpenTelemetry Logs API support to the datadog-opentelemetry crate, mirroring the existing metrics implementation.

Motivation

• Provides a consistent API for logs export matching the metrics API (logs() mirrors metrics())
• Enables OTLP-based logs export via gRPC and HTTP/protobuf protocols
• Follows the same patterns and conventions as the metrics implementation for maintainability

Additional Notes

• New feature flags: logs-grpc and logs-http (mirroring metrics-grpc and metrics-http)
• Configuration via standard OpenTelemetry environment variables (DD_LOGS_OTEL_ENABLED, OTEL_EXPORTER_OTLP_LOGS_*)
• Returns no-op logger provider on errors to prevent application crashes
• Note: OpenTelemetry 0.31.0 doesn't have a global logger provider setter, so users must manage the logger provider instance