Add example demonstrating Nested type serialization issue with custom POJOs

[bk-mz]

This example demonstrates an issue when using the POJO serializer with nested custom objects.

Test Case

Schema:

address Nested(street String, city String)

POJO:

public class ArticleViewEvent {
    private Address address;
}

public class Address {
    private String street;
    private String city;
}

Usage:

Address address = new Address("123 Main St", "San Francisco");
pojoWriter.submit(new ArticleViewEvent(..., address));

Error

WARN com.clickhouse.client.api.serde.POJOSerDe - No getter method found for column: addressstreet
Exception: No serializer found for column 'address.street'. Did you forget to register it?

The POJO serializer looks for getAddressStreet() and getAddressCity() methods (flattened column names) but the POJO only has getAddress() returning an Address object. The serializer doesn't traverse nested object graphs.

To run the example:

docker run -d -p 8123:8123 clickhouse/clickhouse-server
cd examples/client-v2
mvn exec:java -Dexec.mainClass="com.clickhouse.examples.client_v2.Main"

---

Note

Introduces a nested POJO example to exercise ClickHouse Nested/Tuple serialization.

• Adds Address class and embeds it in ArticleViewEvent as address
• Updates table schema in article_view_event_init.sql to include address Nested(street String, city String)
• Modifies Main.java to construct Address and submit it via POJO2DbWriter

This demonstrates the serializer behavior with nested custom objects (per PR description).

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