GH-50628: [C++][Parquet] Rework RecordReader to introduce flat optional optimization #8601

Job	Run time
check-labels / Check labels	4s
check-enabled	3s
matrix.title	-1s
ODBC Windows	-1s
JNI macOS	-1s
ARM64 Windows 11 MSVC	-1s
JNI ${{ matrix.platform.runs-on }} ${{ matrix.platform.arch }}	-1s
JNI Windows	-1s
ODBC ${{ matrix.build-type }} ${{ matrix.architecture }} macOS ${{ matrix.macos-version }}	-1s
ODBC Linux	-1s
ODBC nightly	-1s
report-extra-cpp	-1s
	-3s

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