add confidence interval for item rest correlation (DO NOT MERGE YET) #1336

Job	Run time
unit-tests / scheduled-run-check	1s
unit-tests / ubuntu-latest (R release)	19m 46s
unit-tests / macOS-latest (R release)	5m 43s
unit-tests / windows-latest (R release)	6m 35s
	32m 5s

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