feat: implement client-side caching with Redis invalidation support

Add comprehensive client-side caching functionality that leverages the push notification
infrastructure for automatic cache invalidation.

Core Features:
- Local in-memory cache with configurable size and TTL
- Automatic Redis CLIENT TRACKING integration
- Real-time cache invalidation via push notifications
- LRU eviction policy for memory management
- Thread-safe operations with RWMutex
- Comprehensive statistics and monitoring

API Components:
- ClientSideCache: Main cache implementation
- ClientSideCacheOptions: Configuration options
- Client integration methods: EnableClientSideCache, DisableClientSideCache
- Convenience methods: CachedGet, CachedSet, CachedDel
- Statistics: GetStats with hits, misses, evictions, hit ratio

Implementation Details:
- Uses existing push notification system for invalidation
- Integrates with Redis CLIENT TRACKING (RESP3 required)
- Supports BCAST mode for prefix-based tracking
- Non-blocking invalidation processing
- Graceful fallback to Redis on cache misses
- Automatic cleanup on client close

Benefits:
- Significant performance improvements for read-heavy workloads
- Reduced Redis server load and network traffic
- Automatic cache coherence with real-time invalidation
- Transparent integration with existing Redis operations
- Zero configuration required (sensible defaults)

Test Coverage:
- Comprehensive unit tests for all cache operations
- Integration tests with real Redis instances
- Edge cases: expiration, eviction, invalidation
- Statistics verification and cache management
- Error handling and graceful degradation

Example Usage:
```go
// Enable client-side caching
client.EnableClientSideCache(&redis.ClientSideCacheOptions{
    MaxSize: 1000,
    DefaultTTL: 5 * time.Minute,
})

// Use cached operations
value, err := client.CachedGet(ctx, "key").Result()
err = client.CachedSet(ctx, "key", "value", time.Hour).Err()
```

Files Added:
- client_side_cache.go: Core implementation
- client_side_cache_test.go: Comprehensive tests
- examples/client-side-cache/: Working example with documentation

Integration:
- Leverages existing push notification infrastructure
- Updates shouldSkipNotification filtering (invalidate now processed)
- Maintains backward compatibility
- No breaking changes to existing APIs

examples/client-side-cache/go.mod

@@ -0,0 +1,7 @@
+module client-side-cache-example
+
+go 1.21
+
+require github.com/redis/go-redis/v9 v9.11.0
+
+replace github.com/redis/go-redis/v9 => ../..