fix(agent): Respect processor order in file

[srebhan]

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

resolves #12849

Processor and aggregators are sensitive to order changes in Telegraf as their order defines the actual processing done and the metrics that are taken into account. When loading the configuration files, the order of those plugins is preserved taking the order option into account. However, the agent itself sorts the processors again before actually setting up the processing pipeline. While this on itself does not cause an issue, there is a subtle twist as the "order" is reversed using the sorting. This is necessary because the agent constructs the pipeline from output to input and thus needs to start with the "last" processor.
However, the sorting only affects the processors with an explicit order setting, leaving all other processors in the initial order. As a consequence, those processors will then process in reverse order wrt the appearance in the config file...

This PR removes the superfluous sorting in the agent and takes the config-order as-is. It then adds the processors from output-to-input, i.e. it truly iterates the processors in reverse order, thus keeping the processing order equivalent to the appearance in the file while respecting explicit order statements.

[powersj]

Thank you for putting up the PR!

[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