fix(parsers.json_v2): Check if gpath exists and support optional in fields/tags

[sspaink]

• Updated associated README.md.
• Wrote appropriate unit tests.
• Pull request title or commits are in conventional commit format

resolve: #10670
resolve: #10776

#10468 introduced a bug by checking if a gpath exists by comparing the return type to null, which prevented null being allowed in a JSON file. Instead it should have used the provided Exists() method as suggested in #10670 by @kitlaan and @kgrimsby thanks! 🎉. This PR updates the parser to use Exists() when appropriate. It was also suggested to error only after all the paths have been checked which seems like a good idea, but I am hesitant introducing that into this fix because it does change behavior and maybe keeping this PR just to fix the bug?

#10468 introduced the configuration option optional so that the json_v2 parser can work better with the mqtt_consumer plugin when the paths aren't guaranteed in the JSON. This PR overlooked the global field and tag to include the configuration as well, this was pointed out in #10776. This PR allows setting optional to field and tag as well.

[telegraf-tiger]

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DEB	RPM	TAR GZ	ZIP
amd64.deb	aarch64.rpm	darwin_amd64.tar.gz	windows_amd64.zip
arm64.deb	armel.rpm	darwin_arm64.tar.gz	windows_i386.zip
armel.deb	armv6hl.rpm	freebsd_amd64.tar.gz
armhf.deb	i386.rpm	freebsd_armv7.tar.gz
i386.deb	ppc64le.rpm	freebsd_i386.tar.gz
mips.deb	riscv64.rpm	linux_amd64.tar.gz
mipsel.deb	s390x.rpm	linux_arm64.tar.gz
ppc64el.deb	x86_64.rpm	linux_armel.tar.gz
riscv64.deb		linux_armhf.tar.gz
s390x.deb		linux_i386.tar.gz
		linux_mips.tar.gz
		linux_mipsel.tar.gz
		linux_ppc64le.tar.gz
		linux_riscv64.tar.gz
		linux_s390x.tar.gz
		static_linux_amd64.tar.gz