feat(workflow_engine): Add way to hook logic into `Detector`

[wedamija]

This adds some ways that we can hook custom logic into the detector. Looking at the options, it feels like splitting this logic into different handlers makes sense. Doesn't have to be a function - we can implement classes so that we can share logic.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 96.73913% with 3 lines in your changes missing coverage. Please review.

:white_check_mark: All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/sentry/incidents/grouptype.py	95.23%	1 Missing :warning:
src/sentry/workflow_engine/models/detector.py	97.50%	1 Missing :warning:
src/sentry/workflow_engine/processors/detector.py	95.23%	0 Missing and 1 partial :warning:

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #78540    +/-   ##
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  Coverage   78.25%   78.26%            
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  Files        7105     7107     +2     
  Lines      313227   313368   +141     
  Branches    51135    51161    +26     
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+ Hits       245127   245245   +118     
- Misses      61742    61758    +16     
- Partials     6358     6365     +7