fix(inputs.cisco_telemetry_mdt): Fix MDT source field overwrite

[0x2142]

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

resolves #11920

Incoming Cisco MDT payload may contain source key, which is overwritten upon receipt by Telegraf. This update resolves the issue by renaming the incoming source key if present. By default, this key will be renamed to mdt_source - however, there is an option for the user to manually specify a desired name using the source_field_name config value.

[srebhan]

Thanks for fixing this issue @0x2142! One small comment and I think you should add a unit-test to make sure the source is really stored in the intended way.

[telegraf-tiger]

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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]

Good job @0x2142! Thanks for the fix!