YQL-21315: Implement block variant comp node

#### Implement Block Variant Computational Node ✎

- Introduces a new computational node for handling variant types in block operations, supporting both tuple and struct variants
- Adds type annotation and runtime support for the new variant node with proper type checking and validation
- Implements block-level variant operations that can select specific alternatives from variant types based on index or member name
- Includes comprehensive unit tests and integration tests covering both tuple and struct variant scenarios
- Updates runtime version to 81 and integrates the new node into the compilation and execution pipeline
- Provides SQL syntax support for variant operations through the `Variant` function with appropriate type handling

<a href="https://nda.ya.ru/t/qa0kX64r7DqvtN"><font size="2">Autodescription by Yandex Code Assistant</font></a>
commit_hash:a07c1a365815a271cdd34958369156fd542fd5cc

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