fix(custom_builder): Correctly handle serializers and parsers

[srebhan]

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

resolves #13441

This PR fixes the handling of serializers and solves #13441. During debugging I also noticed that, in case of multiple plugin instances, the default parser and serializer is not correctly selected if one of the other plugins already defines a data_format. Furthermore, processors.execd requires a special treatment as it requires both a serializer and parser of the same type with the defined data_format. To handle those cases correctly the internal code had to be changed quite a bit but is much cleaner and much more robust IMO.

[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

[powersj]

Hmm something is up with the influx_upstream option, this built a custom version with the influx serializer, but then failed to run because influx_upstream of course missing:

[[inputs.exec]]
commands = ["echo metric,tag=1 value=42"]
data_format = "json"

[[outputs.file]]
data_format = "influx_upstream"

[srebhan]

@powersj will take a look...

[srebhan]

@powersj I do not see any influx_upstream serializer... Do you expect me to output a warning or something?

[powersj]

Do you expect me to output a warning or something?

facepalm - it's a parser not a serializer.... let me give this one final look over