[client-v2] Variant Type Support

[chernser]

Description

Variant type example definition:

CREATE TABLE external_projects ( id Variant(UUID, UInt32, String), name String) Engine=MergeTree ORDER BY ()

Binary Encoded as: [ord number of type from definition when types are sorted alphabetically (String = 0)][... encoded value of selected type]

This type is similar to Tuple, but only one value is present at a time.
Variants can be accessed by sub column names. This should be implemented as ability of ClickHouseBinaryReader:

      // for query select id, name from external_projects 
      String id = reader.getString("id.String"); // should return string value or throw exception that value missing or null if nullable column
      String id = reader.getInteger("id.String"); // should throw exception about type mismatch even if column is nullable

There should be possibility to get variant type. This requires to add getValueType method.

When encoding POJO:

• Most suitable type is selected
• Numbers are not converted to strings
• Strings not converted to DateTime values and vice versa

Variant Type Documentation