[C++] Replace Extension RapidJSON usage with simdjson #50873

Describe the enhancement requested

The Arrow extension module contains RapidJSON usage for JSON parsing and deserialization under cpp/src/arrow/extension.

Replace the remaining RapidJSON usage with the existing simdjson helpers and On-Demand API, migrating the extension components incrementally while preserving existing behavior and validation.

Additional context

Parent Issue: GH-35460.

Component(s)

C++