feat: add featureUsageCallback and fix trackingCallback exception handling

[vazarkevych]

Summary

• Fixes Exception should NOT be swallowed if the logger is not set #22: re-throw exception in trackingCallback when logger is not set (previously swallowed silently)
• Adds featureUsageCallback — fires on every getFeature() call, mirrors TypeScript onFeatureUsage
• Fixes trackingCallback deduplication — callback no longer fires twice for the same user/experiment/variation

Changes

• src/Growthbook.php: re-throw in catch block if $this->logger is null
• src/Growthbook.php: add featureUsageCallback with deduplication via trackedFeatures
• src/Growthbook.php: add trackingCallback deduplication using hashAttribute+hashValue+experimentKey+variationId key
• tests/GrowthbookTest.php: add tests for exception re-throw with and without logger
• README.md: document featureUsageCallback