Optional user-supplied key/value pairs in output

[logorrheic]

The user / configuration management system can add additional key/value
pairs which can add useful contextual information to the JSON records
produced.

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• I've been mindful about doing atomic commits, adding documentation to my changes, not refactoring too much.
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• I've written tests to cover the new code and functionality included in this PR.
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PR Summary

Adds the ability to add arbitrary key/value pairs to every output record. This capability can add extra contextual information that may be useful for systems processing and classifying audit records.

Related Issues

N/A

Test strategy

Basic test added.

[CLAassistant]

All committers have signed the CLA.