Support the use of unions in subqueries #348

Job	Run time
pure-fluent-integration-test	2m 1s
integration-tests (swift:5.10-jammy, sqlite-kit, fluent-sqlite-driver)	3m 34s
integration-tests (swift:5.10-jammy, mysql-kit, fluent-mysql-driver)	5m 38s
integration-tests (swift:5.10-jammy, postgres-kit, fluent-postgres-driver)	6m 30s
unit-tests / api-breakage	2m 1s
unit-tests / macos-unit (macos-13, ~15.2)	2m 9s
unit-tests / linux-unit (swift:5.8-jammy)	1m 25s
unit-tests / macos-unit (macos-14, latest)	2m 20s
unit-tests / linux-unit (swift:5.9-jammy)	1m 41s
unit-tests / linux-unit (swift:5.10-jammy)	1m 39s
unit-tests / linux-unit (swiftlang/swift:nightly-6.0-jammy)	2m 7s
unit-tests / linux-unit (swiftlang/swift:nightly-main-jammy)	2m 8s
	33m 13s

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