[Backport 2.x] add a feature that flattens custom result index when enabled (#1401) #1151

Job	Run time
spotless	47s
spotless	47s
Get-CI-Image-Tag / Get-CI-Image-Tag	7s
Get-CI-Image-Tag / Get-CI-Image-Tag	7s
Build and Test Anomaly detection Plugin on Linux (11)	30m 24s
Build and Test Anomaly detection Plugin on Linux (11)	48m 24s
Build and Test Anomaly detection Plugin on MacOS (11)	58m 40s
Build and Test Anomaly detection Plugin on MacOS (11)	58m 40s
Build and Test Anomaly Detection Plugin on Windows (11)	51m 56s
Build and Test Anomaly Detection Plugin on Windows (11)	51m 56s
Build and Test Anomaly detection Plugin on Linux (17)	47m 57s
Build and Test Anomaly detection Plugin on Linux (17)	47m 57s
Build and Test Anomaly detection Plugin on MacOS (17)	59m 24s
Build and Test Anomaly detection Plugin on MacOS (17)	59m 24s
Build and Test Anomaly Detection Plugin on Windows (17)	52m 36s
Build and Test Anomaly Detection Plugin on Windows (17)	52m 36s
Build and Test Anomaly detection Plugin on Linux (21)	34m 31s
Build and Test Anomaly detection Plugin on Linux (21)	49m 45s
Build and Test Anomaly Detection Plugin on Windows (21)	52m 57s
Build and Test Anomaly Detection Plugin on Windows (21)	52m 57s
Build and Test Anomaly detection Plugin on MacOS (21)	59m 27s
Build and Test Anomaly detection Plugin on MacOS (21)	59m 27s
	15h 30m 46s

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