Move LoadPromotions out of OrderAdjuster namespace

[mamhoff]

Summary

Loading the currently available promotions can be quite useful outside of the order adjuster. This moves the LoadPromotions class out of there and into the main SolidusPromotions namespace so that it's clear that this class can be used elsewhere.

Extracted from #6287

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[codecov[bot]]

Codecov Report

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

Project coverage is 88.98%. Comparing base (bbca5cc) to head (987fd44). Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6292      +/-   ##
==========================================
+ Coverage   86.65%   88.98%   +2.32%     
==========================================
  Files         521      863     +342     
  Lines       11943    18425    +6482     
==========================================
+ Hits        10349    16395    +6046     
- Misses       1594     2030     +436     

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