What this PR does / why we need it:

Optimizes the DynamoDB online store implementation to reduce online feature serving latency.

1. O(1) dictionary lookup instead of O(n log n) sorting - _process_batch_get_response now uses dictionary-based lookup for response ordering instead of sorting.

2. Cached TypeDeserializer - Added class-level cached TypeDeserializer instance to avoid per-request object instantiation overhead in async reads.

3. Entity ID computation caching - _to_entity_ids now caches computed entity IDs within a request to avoid redundant hashing for duplicate entity keys.

4. VPC endpoint support for async client - The endpoint_url config is now properly passed to the async aiobotocore client, enabling DynamoDB VPC endpoints for reduced network latency.

5. Improved default configuration values:

   • batch_size: 40 → 100 (max allowed by DynamoDB BatchGetItem)
   • max_pool_connections: 10 → 50 (better concurrency)
   • keepalive_timeout: 12s → 30s (better connection reuse)
   • connect_timeout: 60s → 5s (faster failure detection)
   • read_timeout: 60s → 10s (faster failure detection)
   • total_max_retry_attempts: None → 3 (bounded retries)
   • retry_mode: None → "adaptive" (smart retry with rate limiting)

6. Pre-allocated result lists - Response processing now pre-allocates result lists instead of using append-based growth.