WIP Extract legacy promotions

[mamhoff]

Summary

This extracts the legacy promotion system into a gem that lives within the solidus codebase. I've talked about this with @kennyadsl as a way to integrate a new solidus_promotions gem later, modeled after solidus_friendly_promotions.

This is still Work in progress, and comments are welcome.

Checklist

The following are mandatory for all PRs:

• [ ] I have written a thorough PR description.
• [ ] I have kept my commits small and atomic.
• [ ] I have used clear, explanatory commit messages.

The following are not always needed:

• 📖 I have updated the README to account for my changes.
• 📑 I have documented new code with YARD.
• 🛣️ I have opened a PR to update the guides.
• ✅ I have added automated tests to cover my changes.
• 📸 I have attached screenshots to demo visual changes.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 93.02326% with 6 lines in your changes missing coverage. Please review.

Project coverage is 88.94%. Comparing base (97f9f69) to head (a78e28d).

Files	Patch %	Lines
...otions/migrations/promotions_with_code_handlers.rb	0.00%	3 Missing :warning:
...s_legacy_promotions/testing_support/factory_bot.rb	76.92%	3 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5634      +/-   ##
==========================================
+ Coverage   88.92%   88.94%   +0.01%     
==========================================
  Files         719      724       +5     
  Lines       16944    16974      +30     
==========================================
+ Hits        15068    15097      +29     
- Misses       1876     1877       +1     

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[mamhoff]

Depends on #5635

[jarednorman]

Thanks for your work on this @mamhoff. Now to find time to actually review it properly... 😅

[tvdeyen]

@kennyadsl are we ready to go?