Better typing for PluginLoader#loadPlugin(..)

[andrebrait]

• Follow up to Fixes #3219: Add support for static mocks on DoNotMockEnforcer #3220
• Make the method return the lowest common denominator type

Checklist

• Read the contributing guide
• PR should be motivated, i.e. what does it fix, why, and if relevant how
• If possible / relevant include an example in the description, that could help all readers
  including project members to get a better picture of the change
• Avoid other runtime dependencies
• Meaningful commit history ; intention is important please rebase your commit history so that each
  commit is meaningful and help the people that will explore a change in 2 years
• The pull request follows coding style
• Mention Fixes #<issue number> in the description if relevant
• At least one commit should mention Fixes #<issue number> if relevant

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (9173522) 85.33% compared to head (85a74d1) 85.33%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #3242   +/-   ##
=========================================
  Coverage     85.33%   85.33%           
  Complexity     2923     2923           
=========================================
  Files           336      336           
  Lines          8905     8906    +1     
  Branches       1108     1108           
=========================================
+ Hits           7599     7600    +1     
  Misses         1013     1013           
  Partials        293      293           

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[TimvdLippe]

Nice, thank you!