Add structured Logs / OTLP log capture support

[marandaneto]

Summary

Add structured Logs / OTLP log capture support to the PHP SDK.

Spec

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

Current evidence

The SDK audit found no captureLog, logger helper, OTLP encoding, log batcher, or /i/v1/logs transport.

Expected behavior

Implement the canonical logs behavior, 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.