Implement Ser/De for Avro Union and ClickHouse Variant Types

Why it matters

This feature addresses the need to seamlessly convert data between Avro's Union type and ClickHouse's flexible Variant type, enabling better compatibility and easier integration for users working with Avro-formatted data in ClickHouse.

How to use it

Users can leverage this feature automatically when importing or exporting data between Avro and ClickHouse formats. The ser/de functionality enables smooth conversion without additional configuration, making it transparent during data ingestion or query operations involving Variant columns and Avro Union data.