Add fixed shape tensor

[Alex-PLACET]

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[Copilot]

Pull request overview

This PR adds support for the Arrow canonical extension type arrow.fixed_shape_tensor, which enables storing fixed-shape tensors in Arrow format. The implementation follows the Apache Arrow specification for fixed shape tensor arrays.

Key Changes

• Introduces fixed_shape_tensor_array class wrapping fixed_sized_list_array with tensor-specific metadata
• Implements JSON serialization/deserialization for tensor metadata (shape, dimension names, permutation)
• Replaces nlohmann_json dependency with simdjson for improved JSON parsing performance
• Adds comprehensive test coverage with 752 lines of tests

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
include/sparrow_extensions/fixed_shape_tensor.hpp	Defines the fixed_shape_tensor_array class and metadata structures with comprehensive documentation
src/fixed_shape_tensor.cpp	Implements metadata validation, JSON serialization, and array construction logic
tests/test_fixed_shape_tensor.cpp	Provides extensive test coverage for metadata operations, array construction, and spec compliance
cmake/external_dependencies.cmake	Adds simdjson dependency and updates doctest version
environment-dev.yml	Replaces nlohmann_json with simdjson in conda dependencies
CMakeLists.txt	Registers new source files and headers in build system
include/sparrow_extensions.hpp	Includes the new fixed_shape_tensor header in main library header
tests/CMakeLists.txt	Adds test file to test suite

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