Allow setting a different user class for backend users

[elia]

Disclaimer: this PR was entirely written by codex under my guidance, for good or for bad 😊

Summary

This change will open to having separate classes for customers and backend operators, this brings the following advantages:

• Security It's impossible to do privilege escalation if the two sets are siloed
• Authentication most of the time authentication rules for "admins" and customers are quite different, e.g. domain based Google signup for admins, vs. magic link for customers, etc.
• Impersonation impersonating a customer becomes way easier if you can overlap the two authentication systems

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[codecov]

Codecov Report

❌ Patch coverage is 65.71429% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.31%. Comparing base (d070f94) to head (6d2dfbe).

Files with missing lines	Patch %	Lines
core/lib/spree/core.rb	33.33%	6 Missing ⚠️
core/lib/spree/testing_support/capybara_ext.rb	0.00%	5 Missing ⚠️
...app/controllers/solidus_admin/orders_controller.rb	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6329      +/-   ##
==========================================
- Coverage   89.35%   89.31%   -0.05%     
==========================================
  Files         961      961              
  Lines       20195    20210      +15     
==========================================
+ Hits        18046    18051       +5     
- Misses       2149     2159      +10     

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[jarednorman]

This seems like a good idea to me. This separation could be useful for some marketplace stores we work on as well.

[elia]

@jarednorman wanted to do this for a long time, and never got the time until I was able to delegate 😅
If there's buy in I'll give it some more (compute) time and finish this first iteration, later I think we can introduce to a better default for new apps.

cc @rainerdema @spaghetticode