Make promotion handler classes configurable

[mamhoff]

Summary

We have four promotion handlers in the codebase, but only two are actually used: the promotion handler "coupon" and the promotion handler "shipping". If one chose to use a promotion system that works different from the stock Solidus one, these two promotion handlers must be configurable.

See also friendlycart/solidus_friendly_promotions#61 because this extension point does not exist yet.

Checklist

• I have written a thorough PR description.
• I have kept my commits small and atomic.
• I have used clear, explanatory commit messages.
• 📑 I have documented new code with YARD.
• ✅ I have added automated tests to cover my changes.

[codecov]

Codecov Report

Merging #5466 (1ceffb6) into main (b653f69) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #5466   +/-   ##
=======================================
  Coverage   88.96%   88.97%           
=======================================
  Files         617      617           
  Lines       14876    14878    +2     
=======================================
+ Hits        13235    13237    +2     
  Misses       1641     1641           

Files	Coverage Δ
...p/controllers/spree/api/coupon_codes_controller.rb	100.00% <100.00%> (ø)
core/app/models/spree/order.rb	94.65% <100.00%> (ø)
core/lib/spree/app_configuration.rb	99.28% <100.00%> (+0.01%)	⬆️

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[elia]

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