Make the number of workers processing federated query configurable #10793

Job	Run time
lint	4m 35s
build	4m 14s
test	16m 0s
CodeQL	6m 50s
test-no-race	7m 33s
integration-configs-db	2m 45s
integration (requires_docker)	7m 16s
integration (integration_alertmanager)	4m 12s
integration (integration_backward_compatibility)	10m 27s
integration (integration_memberlist)	7m 51s
integration (integration_querier)	15m 23s
integration (integration_ruler)	5m 47s
integration (integration_query_fuzz)	5m 2s
deploy_website	0s
deploy	0s
	1h 37m 55s

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