feat(parsers.value): Add support for automatic fallback for numeric types

[srebhan]

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

This PR adds auto_integer and auto_float options to the value parser. Those two options allow to fallback to string if the parsing into the numeric types fails. This is helpful when the data-source produces multiple streams with mixed numeric and string data such as subscriptions to multiple MQTT topics.

[telegraf-tiger]

Download PR build artifacts for linux_amd64.tar.gz, darwin_amd64.tar.gz, and windows_amd64.zip.
Downloads for additional architectures and packages are available below.

☺️ This pull request doesn't significantly change the Telegraf binary size (less than 1%)

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Artifact URLs

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_arm64.zip
armel.deb	armv6hl.rpm	freebsd_amd64.tar.gz	windows_i386.zip
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

[maf-soft]

I'm a first time user of telegraf and influx and tried to get mqtt to work. I spent about 6 hours investigating why telegraf doesn't write any data, activated debug, much later also found the trace option, read all documentation back and forth, asked others, googled, reinstalled influx multiple times, it was really frustrating.
The documentation clearly says auto_float is the way to go. I had no idea that this could be a new feature not yet included in the latest release. Why is there no error message anywhere?
Maybe it's lack of experience on my side, but I would say the documentation should not be updated before the release or clearly mention it.

[srebhan]

@maf-soft sorry for the bad experience! The unfortunate truth is that the parameter value was not checked previously (also fixed with this commit)... Please apologize for the inconvenience and please also feel free to ask on our Slack channel or forum...