Add TaxonRevenue condition

[mamhoff]

Summary

This condition adds a promotion condition that allows store owners to trigger a benefit if the revenue from a defined set of taxons exceeds some value. For example, with this condition we can do things like: If you spend 150 USD on pants, you get a free belt.

Checklist

• I agree that my PR will be published under the same license as Solidus.
• I have written a thorough PR description.
• I have kept my commits small and atomic.
• I have localized any and all user-facing strings that I added to the source code.
• 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]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.68%. Comparing base (c7e850d) to head (2c05beb).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6492   +/-   ##
=======================================
  Coverage   89.68%   89.68%           
=======================================
  Files         992      993    +1     
  Lines       20845    20863   +18     
=======================================
+ Hits        18694    18712   +18     
  Misses       2151     2151           

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[AlistairNorman]

Nice, seems like a useful condition to have!