Event confidence

[TheTechromancer]

This PR modifies event confidence to be a value between 1 and 100. It also adds a cumulative_confidence attribute which will take into consideration the confidence of parents.

E.g. an event with a confidence of 50, if its parent's confidence was also 50, would have a cumulative_confidence of 25.

# override the confidence (by default it's 100)
async def handle_event(self, event):
    await self.emit_event("evilcorp.com", "DNS_NAME", parent=event, confidence=90)

# filter out overly speculative events
async def filter_event(self, event):
    if event.cumulative_confidence < 50:
        return False, "Event's confidence is too low"
    return True

Addresses #1647.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93%. Comparing base (4279621) to head (b59dfd9).
Report is 44 commits behind head on dev.

Additional details and impacted files

@@          Coverage Diff          @@
##             dev   #1655   +/-   ##
=====================================
+ Coverage     93%     93%   +1%     
=====================================
  Files        341     341           
  Lines      25807   25885   +78     
=====================================
+ Hits       23782   23859   +77     
- Misses      2025    2026    +1     

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