Refactor "current_api_user" into instacached helper

[mamhoff]

Summary

In the same spirit as #6245 , which refactored @current_user_roles into a helper method, we do the same for the current_api_user.

Depends on #6245, extracted from #6240.

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[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.82%. Comparing base (f0a7456) to head (5dad30f).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
api/app/controllers/spree/api/base_controller.rb	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6246      +/-   ##
==========================================
- Coverage   88.82%   88.82%   -0.01%     
==========================================
  Files         850      850              
  Lines       18334    18337       +3     
==========================================
+ Hits        16286    16288       +2     
- Misses       2048     2049       +1     

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