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

### 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++