Add extension point: Promotion finder

[mamhoff]

Summary

The API gem references the promotion system in very few places, namely in the promotions endpoint. This endpoint is only accessible to admins, and our admin doesn't actually check it, so another option would be to remove the endpoint entirely.

However, it's simple enough to create a class that does what we need here.

Checklist

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.87%. Comparing base (49e3da4) to head (2f29de0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5677      +/-   ##
==========================================
+ Coverage   88.86%   88.87%   +0.01%     
==========================================
  Files         695      697       +2     
  Lines       16589    16599      +10     
==========================================
+ Hits        14741    14752      +11     
+ Misses       1848     1847       -1     

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