Add core SDK structured Logs public API

[marandaneto]

Summary

Add core SDK structured Logs public API to posthog-ruby.

Spec

• sdk-specs/openspec/specs/logs/spec.md

Current evidence

Rails-only logs support exists, but the core SDK audit found no capture_log / logger public API in PostHog::Client or the public API snapshot.

Expected behavior

Implement logs support at the core SDK level, including:

• Public log capture API / logger helper.
• Severity mapping.
• OTLP log record/envelope encoding.
• Batching/flush behavior for logs.
• Transport to {host}/i/v1/logs?token={projectApiKey}.
• Tests for core spec scenarios independent of Rails.