[C++] Replace OpaqueType RapidJSON usage with simdjson #50904

Describe the enhancement requested

OpaqueType uses RapidJSON for JSON parsing and deserialization under cpp/src/arrow/extension.

Replace the RapidJSON usage with the existing simdjson helpers and On-Demand API while preserving the existing validation and deserialization behavior.

Additional context

This is a sub-task of GH-50873.

Parent Issue: GH-35460.

Component(s)

C++