[C++] Introduce JsonWriter and migrate integration JSON writer #50567

Describe the enhancement requested

Introduce a reusable JsonWriter abstraction based on simdjson's builder API and migrate the integration JSON writer to use it instead of RapidJSON's writer.

Scope:

• Add a reusable JsonWriter wrapper around simdjson's builder API.
• Migrate the integration JSON writer implementation to JsonWriter.
• Update integration tests to use JsonWriter.
• Add unit tests for JsonWriter.

Parent issue: #35460

Component(s)

C++