GH-50913: [C++][Dataset] Replace RapidJSON with JsonWriter

[Reranko05]

Rationale for this change

This PR continues the RapidJSON → simdjson migration by replacing the remaining RapidJSON usage in the Dataset JSON test with Arrow's existing JsonWriter.

Changes

• Replace rapidjson::Writer and rapidjson::OStreamWrapper with arrow::json::JsonWriter.
• Replace RapidJSON-specific SizeType usage with the JsonWriter interface.
• Preserve the existing newline-delimited JSON test fixture generation.
• Remove the unused RapidJSON dependency from the Dataset test in CMake and Meson.
• Preserve the existing Dataset JSON test coverage and behavior.

Fixes: #50913

• GitHub Issue: [C++][Dataset] Replace RapidJSON with JsonWriter in JSON Dataset #50913

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[kou]

Can we fix the Meson job failure as a separated task to test Meson related changes in this PR before we proceed this PR?

https://github.com/apache/arrow/actions/runs/32293675583/job/96201011745?pr=50914#step:6:1103

FAILED: [code=1] subprojects/bzip2-1.0.8/libbz2.a.p/bzlib.c.o 
/usr/local/bin/sccache cc -Isubprojects/bzip2-1.0.8/libbz2.a.p -Isubprojects/bzip2-1.0.8 -I../../arrow/cpp/subprojects/bzip2-1.0.8 -fdiagnostics-color=always -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -Wextra -std=c11 -O0 -g -fPIC -MD -MQ subprojects/bzip2-1.0.8/libbz2.a.p/bzlib.c.o -MF subprojects/bzip2-1.0.8/libbz2.a.p/bzlib.c.o.d -o subprojects/bzip2-1.0.8/libbz2.a.p/bzlib.c.o -c ../../arrow/cpp/subprojects/bzip2-1.0.8/bzlib.c
../../arrow/cpp/subprojects/bzip2-1.0.8/bzlib.c: In function 'default_bzalloc':
../../arrow/cpp/subprojects/bzip2-1.0.8/bzlib.c:102:31: warning: unused parameter 'opaque' [-Wunused-parameter]
  102 | void* default_bzalloc ( void* opaque, Int32 items, Int32 size )
      |                         ~~~~~~^~~~~~
../../arrow/cpp/subprojects/bzip2-1.0.8/bzlib.c: In function 'default_bzfree':
../../arrow/cpp/subprojects/bzip2-1.0.8/bzlib.c:109:29: warning: unused parameter 'opaque' [-Wunused-parameter]
  109 | void default_bzfree ( void* opaque, void* addr )
      |                       ~~~~~~^~~~~~
../../arrow/cpp/subprojects/bzip2-1.0.8/bzlib.c: In function 'bzopen_or_bzdopen':
../../arrow/cpp/subprojects/bzip2-1.0.8/bzlib.c:1431:12: error: implicit declaration of function 'fdopen'; did you mean 'fopen'? [-Wimplicit-function-declaration]
 1431 |       fp = fdopen(fd,mode2);
      |            ^~~~~~
      |            fopen
...