Add support for reflect-cpp's JSON #742

Job	Run time
root-hint-install-linux	13s
custom-install-linux	10s
no-base64	8s
with-hunter	3m 19s
preset-support-but-not-enough	1m 12s
no-pico	9s
custom-install-win	1m 58s
default-win	1m 58s
default-linux-with-examples	12s
min-req	1m 29s
default-linux	12s
with-wolfssl	1m 5s
with-libressl	1m 29s
	13m 34s

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