Ensure array_type has front() method + fix array_type on jsoncpp #595

Job	Run time
default-linux	11s
custom-install-linux	10s
default-linux-with-examples	11s
default-win	2m 59s
min-req	8m 59s
preset-support-but-not-enough	11m 34s
custom-install-win	2m 37s
no-base64	17s
no-pico	6s
root-hint-install-linux	10s
with-hunter	3m 7s
with-libressl	1m 22s
with-wolfssl	49s
	32m 32s

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