Make the number of workers processing federated query configurable #10807

Job	Run time
CodeQL	6m 44s
build	4m 2s
lint	4m 21s
test	15m 44s
test-no-race	7m 32s
integration-configs-db	2m 43s
integration (requires_docker)	7m 22s
integration (integration_alertmanager)	3m 59s
integration (integration_backward_compatibility)	11m 12s
integration (integration_memberlist)	7m 47s
integration (integration_querier)	15m 18s
integration (integration_ruler)	5m 44s
integration (integration_query_fuzz)	5m 0s
deploy_website	0s
deploy	0s
	1h 37m 28s

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