Legacy Promotions: Add inverse_of/dependent options

[mamhoff]

Summary

Adds inverse_of and dependent options to has_many relations in solidus_legacy_promotions. I'm not adding foreign keys and all that here because technically this gem won't be supported for much longer.

Extracted from #6240

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.95%. Comparing base (acb9cc8) to head (b681cac).
Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6273   +/-   ##
=======================================
  Coverage   88.95%   88.95%           
=======================================
  Files         860      860           
  Lines       18422    18422           
=======================================
  Hits        16388    16388           
  Misses       2034     2034           

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