Add mutant

[mbj]

This shows a basic mutant setup on the core component of solidus, focusing on postgresql DB driver.

Covering

• booting a rails app mutant aware
• DB concurrency setup.

Left out intentionally are:

• DB cleaner improvements leveraging the dynamically cloned databases (that would speed up mutation testing a lot)
• CI integration

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.68%. Comparing base (4e00cb1) to head (a7c1d54).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6489   +/-   ##
=======================================
  Coverage   89.68%   89.68%           
=======================================
  Files         992      992           
  Lines       20846    20846           
=======================================
  Hits        18695    18695           
  Misses       2151     2151           

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