Summary

Adds DynamoDB-specific list update functionality to avoid inefficient read-modify-write patterns for array-based features. This implements the solution requested in #5687.

Fixes #5687

Changes

• Add update_online_store() and update_online_store_async() methods to DynamoDBOnlineStore class
• Add corresponding methods to FeatureStore class for user-facing API
• Support DynamoDB UpdateItem with list_append expressions for efficient array operations
• Implement proper list serialization for all ValueProto list types
• Add comprehensive unit tests for both DynamoDB online store and FeatureStore methods
• Maintain 100% backward compatibility with existing write_to_online_store behavior
• DynamoDB-specific functionality kept separate from core OnlineStore interface

Testing

• Added comprehensive unit tests for DynamoDB online store update methods
• Added FeatureStore integration tests for both sync and async methods
• All tests pass and maintain backward compatibility.

Usage Example

# Append new transactions to existing transaction history
store.update_online_store(
    feature_view_name="user_transactions",
    df=new_transactions_df,
    update_expressions={
        "transaction_history": "list_append(transaction_history, :new_val)",
        "recent_amounts": "list_append(:new_val, recent_amounts)[:10]"  # sliding window
    }
)


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