feat(inputs.netflow): Add netflow plugin

[srebhan]

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

resolves #174

This PR adds a netflow input plugin, acting as a collector for Netflow v5, Netflow v9 and IPFIX data. Additional protocol versions and custom field mappings might be added at a later point.

[iec61131]

Awesome PR! @srebhan Sven , what is the ETA to merge this in master ?

[srebhan]

@iec61131 please discuss this in issue #174. I hope to finish the IPFIX mapping within the next two weeks and hope to get this into 1.25 scheduled for mid of December. Anyway, feedback on the element-id -- field mappings is appreciated!

[powersj]

This looks great Sven - a few questions and request for additional tests

[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_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
		static_linux_amd64.tar.gz