[WIP] GH-50873: [C++] Migrate FixedShapeTensorType deserialization to simdjson

[Reranko05]

Rationale for this change

This PR continues the simdjson migration by replacing RapidJSON usage under cpp/src/arrow/extension with simdjson and the existing simdjson_internal helpers. It also updates the associated tests to reflect the simdjson-based JSON type handling.

WIP: #50873

[github-actions]

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