feat(outputs.mqtt): Enhance routing capabilities

[serroba]

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

resolves #12221

The objective was to add more flexibility to the MQTT output plugin. The work in this PR adds placeholders and effective support for:

• backward compatibility behaviour and respect default previous topic name
• adds support to add hard-coded strings as part of the topic
• by extension of the above, adds support to use environment variables
• adds support to use tags as part of the topic name.

---

Not sure if this helps, but here some evidence of the results after some local testing:

[serroba]

@srebhan thank you very much for your review and time dedicated to provide good feedback. I do wonder still though if you have some advice WRT the failing integration test. It seems unrelated to my changes. Is this due to some flakiness on the suite? or should I fix something on my end?

[serroba]

I have re-tested after the refactor to use the go-template approach

Things are still working as expected and I believe the code is much simpler now

The local topic I run was:

topic = '{{ .TopicPrefix }}/fixed-value/{{ .Hostname }}/{{ .PluginName }}/{{ .Tag "slave_id" }}/{{ .Tag "name" }}/${TERM_SESSION_ID}/<pluginname>/{{ .Tag "invalid" }}'

[serroba]

@srebhan @powersj is there anything else I can do to help with the review? Thanks in advance

[serroba]

@srebhan I was wondering if you have any further feedback on this PR, I'd appreciate your thoughts

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ppc64el.deb	x86_64.rpm	linux_armel.tar.gz
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s390x.deb		linux_i386.tar.gz
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[serroba]

Thank you for all your help @srebhan

[serroba]

Thank you @srebhan for all the support and helping make this happen 🙇