Support creating an aggregate using arbitrary SELECT statements #2004

Job	Run time
Test with ECJ	1m 59s
Integration Test (h2)	1m 54s
Integration Test (mysql)	3m 0s
Integration Test (mysql8)	3m 11s
Build	2m 47s
Integration Test (postgresql)	2m 27s
Integration Test (sqlite)	1m 59s
Integration Test (sqlserver)	3m 5s
Integration Test (oracle)	5m 45s
Publish	0s
	26m 7s

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