Add 'path' attribute for FINDING and VULNERABILITY

[TheTechromancer]

This PR adds a path attribute to FINDING and VULNERABILITY, and also allows these events to skip specifying a host. When no host is specified, it will automatically walk up the chain of parents and use the closest available one. This will be more convenient for modules that are raising FINDINGs from events without hosts, like FILESYSTEM.

In response to #1636.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93%. Comparing base (4c9fbdf) to head (35a737b).
Report is 27 commits behind head on dev.

Additional details and impacted files

@@          Coverage Diff          @@
##             dev   #1656   +/-   ##
=====================================
+ Coverage     93%     93%   +1%     
=====================================
  Files        341     341           
  Lines      25807   25842   +35     
=====================================
+ Hits       23783   23819   +36     
+ Misses      2024    2023    -1     

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