Add variable shape tensor

[Alex-PLACET]

No description provided.

[Copilot]

Pull request overview

This PR implements the Variable Shape Tensor Array extension following the Apache Arrow canonical extension specification. The implementation allows efficient storage and transfer of multi-dimensional tensors with varying shapes.

Key changes include:

• Addition of variable_shape_tensor_array class with metadata serialization/deserialization
• Implementation of Arrow extension metadata handling and validation
• Migration from nlohmann_json to simdjson for improved JSON parsing performance

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/test_variable_shape_tensor.cpp	Comprehensive test suite for metadata validation, JSON serialization, and round-trip operations
tests/CMakeLists.txt	Registers new test file in the build system
src/variable_shape_tensor.cpp	Core implementation of metadata, serialization, and array construction
include/sparrow_extensions/variable_shape_tensor.hpp	Header defining public API and template implementations
include/sparrow_extensions.hpp	Adds variable_shape_tensor header to main include
environment-dev.yml	Updates dependency from nlohmann_json to simdjson
docs/source/variable_shape_tensor_array.md	Comprehensive documentation with usage examples and API reference
docs/source/main_page.md	Adds documentation link for fixed_shape_tensor_array
cmake/external_dependencies.cmake	Adds simdjson dependency configuration
CMakeLists.txt	Registers variable_shape_tensor.cpp in build sources

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[JohanMabille]

I think this can be simplified into return m_storage.children_count() == 2 && m_metadata.is_valid();