Common: Allow '.' characters in Event tags

[mssalvatore]

The MITRE ATT&CK framework identifies subtechniques by using a '.' to separate the technique ID from the subtechnique ID. For example, the technique "Defacement" has the ID T1491, whereas the subtechnique "Internal Defacement" has the ID T1491.001.

PR Checklist

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• Was the CHANGELOG.md updated to reflect the changes?
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Testing Checklist

• Added relevant unit tests?
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  Tested by {Running the Monkey locally with relevant config/running Island/...}

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