[C++] Replace RapidJSON with simdjson in tensor extension types #50901

Describe the enhancement requested

The Arrow extension module uses RapidJSON for JSON deserialization in FixedShapeTensorType and VariableShapeTensorType.

Replace these RapidJSON usages with the existing simdjson helpers and On-Demand API while preserving the existing serialization, validation, and deserialization behavior.

Additional context

This is a focused follow-up to #50873 and part of the broader simdjson migration tracked by #35460.

Component(s)

C++