GH-48316: [C++] Use FetchContent for bundled nlohmann-json #54848

Job	Run time
AMD64 Windows 2022 Python 3.13	8m 4s
ARM64 macOS 14 Python 3	17m 57s
AMD64 macOS 15-intel Python 3	51m 22s
AMD64 Conda Python 3.10 Sphinx & Numpydoc	53s
AMD64 Conda Python 3.10 Pandas 1.3.4	53s
AMD64 Conda Python 3.12 without NumPy	48s
AMD64 Conda Python 3.13 Pandas latest	50s
AMD64 Conda Python 3.11 Without Pandas	46s
AMD64 Conda Python 3.10 Sphinx & Numpydoc	8m 15s
AMD64 Conda Python 3.11 Without Pandas	7m 12s
AMD64 Conda Python 3.13 Pandas latest	7m 33s
AMD64 Conda Python 3.10 Pandas 1.3.4	8m 3s
AMD64 Conda Python 3.12 without NumPy	5m 57s
ARM64 macOS 14 Python 3	17m 57s
AMD64 macOS 15-intel Python 3	51m 22s
AMD64 Windows 2022 Python 3.13	8m 4s
	3h 15m 56s

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