DefaultResultCache::mergeWith() for merging result cache instances

[Slamdunk]

Fix #6078

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.40%. Comparing base (2ad0441) to head (d42717b).
Report is 1 commits behind head on 10.5.

Additional details and impacted files

@@            Coverage Diff            @@
##               10.5    #6081   +/-   ##
=========================================
  Coverage     94.39%   94.40%           
- Complexity     6461     6464    +3     
=========================================
  Files           681      681           
  Lines         20496    20501    +5     
=========================================
+ Hits          19348    19353    +5     
  Misses         1148     1148           

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[sebastianbergmann]

Can you send this for 10.5? It should also "just work" there and if it does, I'll merge it from there to 11.5 and main. Thanks!

[Slamdunk]

Done 😎