Define Propagation Event

[ilija-lazoroski]

What does this PR do?

Fixes part of #2269 .

Add any further explanations here.

PR Checklist

• Have you added an explanation of what your changes do and why you'd like to include them?
• Is the TravisCI build passing?
• Was the CHANGELOG.md updated to reflect the changes?
• Was the documentation framework updated to reflect the changes?
• Have you checked that you haven't introduced any duplicate code?

Testing Checklist

• Added relevant unit tests?
• Have you successfully tested your changes locally? Elaborate:

  Tested by {Running the Monkey locally with relevant config/running Island/...}

• If applicable, add screenshots or log transcripts of the feature working

[codecov]

Codecov Report

Base: 61.20% // Head: 61.22% // Increases project coverage by +0.02% 🎉

Coverage data is based on head (5b9811f) compared to base (25f1230).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head 5b9811f differs from pull request most recent head d1af356. Consider uploading reports for the commit d1af356 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2385      +/-   ##
===========================================
+ Coverage    61.20%   61.22%   +0.02%     
===========================================
  Files          550      551       +1     
  Lines        14410    14419       +9     
===========================================
+ Hits          8819     8828       +9     
  Misses        5591     5591              

Impacted Files	Coverage Δ
monkey/common/agent_events/__init__.py	100.00% <100.00%> (ø)
monkey/common/agent_events/propagation_event.py	100.00% <100.00%> (ø)

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[cakekoa]

Do we want to document this field as well?

[ilija-lazoroski]

As we are overriding the field, I don't think it is really needed.