`DefaultResultCache::mergeWith()` for merging result cache instances #2851

Job	Run time
Dependency Validation	10s
Coding Guidelines	30s
Static Analysis	32s
Unit Tests (ubuntu-latest, 8.3)	14s
Unit Tests (ubuntu-latest, 8.4)	15s
Unit Tests (ubuntu-latest, 8.5)	13s
Unit Tests (windows-latest, 8.3)	37s
Unit Tests (windows-latest, 8.4)	39s
Unit Tests (windows-latest, 8.5)	33s
End-to-End Tests (ubuntu-latest, 8.3)	41s
End-to-End Tests (ubuntu-latest, 8.4)	44s
End-to-End Tests (ubuntu-latest, 8.5)	42s
End-to-End Tests (windows-latest, 8.3)	1m 42s
End-to-End Tests (windows-latest, 8.4)	1m 38s
End-to-End Tests (windows-latest, 8.5)	1m 52s
Code Coverage	2m 22s
Build PHAR	37s
Test PHAR (8.3, pcov)	18s
Test PHAR (8.3, xdebug)	35s
Test PHAR (8.4, pcov)	17s
Test PHAR (8.4, xdebug)	34s
Test PHAR (8.5, pcov)	15s
Test PHAR (8.5, xdebug)	34s
	16m 34s

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