Add support for session window

[petrpan26]

Session Windows Implementation for Quix Streams

This PR introduces Session Windows - dynamic windowing that groups events by activity sessions rather than fixed time intervals.

Key Features

• Activity-driven sessions: Sessions extend automatically when events arrive within timeout period
• Flexible configuration: Configurable timeout and grace periods for late events
• Full aggregation support: Works with all existing aggregation functions (sum, count, mean, max, min, reduce, collect)
• Both modes supported: .current() for real-time updates, .final() for complete sessions
• Closing strategies: Both "key" and "partition" level session expiration

Usage Examples

# User session tracking (30min timeout, 5min grace)
sdf.session_window(timeout_ms=timedelta(minutes=30), grace_ms=timedelta(minutes=5)) \
   .agg(page_views=Count(), actions=Collect("action_type")) \
   .final()

# Real-time fraud detection
sdf.session_window(timeout_ms=timedelta(minutes=10)) \
   .agg(transaction_count=Count(), total_amount=Sum("amount")) \
   .current()

Use Cases Enabled

• User activity tracking (web/app sessions)
• Fraud detection (transaction patterns)
• IoT device monitoring (activity sessions)
• Gaming analytics (player sessions)

Testing & Documentation

• ✅ 32 comprehensive tests covering all session window functionality
• ✅ Complete documentation added to docs/windowing.md with examples
• ✅ Session merging scenarios and edge cases tested
• ✅ Multi-key session management validated

Technical Implementation

• Smart session extension logic with grace period support
• Efficient RocksDB-backed state management
• Proper transaction handling for concurrent access
• Memory-efficient session garbage collection

This enables sophisticated streaming analytics use cases that were previously difficult to implement with fixed-time windows.

[gwaramadze]

Hello. Once again, thanks for your contributions. Here is the first batch of comments. Please take a look. We will continue to review and test it.

[gwaramadze]

Closing this PR as it was superseded by #994