Make the number of workers processing federated query configurable #10794

Job	Run time
CodeQL	6m 22s
lint	4m 17s
build	4m 3s
test	16m 1s
test-no-race	7m 45s
integration-configs-db	2m 53s
integration (requires_docker)	7m 22s
integration (integration_alertmanager)	4m 3s
integration (integration_backward_compatibility)	11m 14s
integration (integration_memberlist)	7m 54s
integration (integration_querier)	15m 7s
integration (integration_ruler)	5m 33s
integration (integration_query_fuzz)	4m 53s
deploy_website	0s
deploy	0s
	1h 37m 27s

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