fix(inputs.netflow): Handle PEN messages correctly

[srebhan]

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

resolves #13305

Messages containing Private Enterprise Numbers (PEN) can define arbitrary elements with IDs colliding with the generally defined element IDs. Therefore, we need to respect the PEN information when doing the lookup in order to not misinterpret elements (e.g. redefined with a different type).

This PR separates the PEN message from the general message lookup by respecting the PEN flag. As we currently have no means in specifying PEN information, those fields will be reported as fields with a type_<pen-number>_<element_id> key and hex-encoded values.
Furthermore, the PR converts a "missing template" error to a warning.

[vknyazhev]

@srebhan I don't understand a bit, do I need to run some file (what?) on the host on which telegraf is running?

[powersj]

@srebhan I'm assigned, but you removed ready for final review? ;)

[srebhan]

@powersj yeah I split out one of the commits into #13325 so I wanted to wait until this is in... Rebase is done now, will re-tag as soon as the tests are through...

[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

[srebhan]

@powersj tests passed...

[powersj]

Thanks!