Add Legacy promotions gem

[mamhoff]

Summary

This adds a new gem to the suite, solidus_legacy_promotions. It's indended to be filled with things from core later (see #5634 ) for what it'll look like).

I would like this scaffold to be around so I can slowly chip away at the commits in #5634 without creating unreviewable PRs for the maintainers.

Checklist

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.86%. Comparing base (49e3da4) to head (3222e60).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5678   +/-   ##
=======================================
  Coverage   88.86%   88.86%           
=======================================
  Files         695      697    +2     
  Lines       16589    16597    +8     
=======================================
+ Hits        14741    14749    +8     
  Misses       1848     1848           

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[kennyadsl]

Thanks for splitting the work this way Martin, it's super helpful!