Support variant type

[chernser]

Summary

This PR is only adds support in Client v2 for Variant Type:

• Reading with POJO and Binary readers
• Writing POJO

TODO in next PR:

• remember variant type ord. num. match for array class
• remember variant type ord. num. match for map key/value class

Checklist

Delete items not relevant to your PR:

• Closes [client-v2] Variant Type Support  #1926
• Unit and integration tests covering the common scenarios were added
• A human-readable description of the changes was provided to include in CHANGELOG
• For significant changes, documentation in https://github.com/ClickHouse/clickhouse-docs was updated with further explanations or tutorials