Performance improvements and enhanced error handling

[RamiAhmed]

Summary

Performance optimizations and improved error handling for production Unity applications.

Changes

• ✅ Memory optimization: Use ListPool and DictionaryPool to reduce GC allocations
• ✅ Async improvements: Flush() now returns Task with CancellationToken support throughout the call chain
• ✅ Better error handling: Comprehensive HTTP status code handling with actionable error messages

Technical Details

// Before: new allocations every time
var props = new Dictionary<string, object>();

// After: pooled allocations
var props = DictionaryPool<string, object>.Get();
// ... use ...
DictionaryPool<string, object>.Release(props);

// Cancellation support
await Aptabase.Flush(cancellationToken);

Testing

✅ Verified memory reduction via Unity Profiler
✅ Tested cancellation token propagation
✅ 100% backward compatible - no breaking changes

Benefits

• Reduced GC spikes in high-frequency event tracking
• Better async/await patterns for Unity lifecycle
• Clear, actionable error messages for debugging
• Improved production readiness

[RamiAhmed]

Hey @NikTheNak @revmarcuspeter-cyber (other devs?)

I'm not able to merge this PR yet (I don't have a 'merge' button?). Are more reviewers required, or?

[RamiAhmed]

I've snuck in another minor addition - support for setting a "response listener" - we wanted to be able to disable our analytics service if we get a 'too many requests' response to avoid spamming the Aptabase servers and save on our own logging and other overhead.