Make the number of workers processing federated query configurable #10800

Job	Run time
CodeQL	6m 27s
lint	4m 19s
test-no-race	7m 18s
build	4m 15s
test	16m 1s
integration (requires_docker)	7m 21s
integration-configs-db	2m 57s
integration (integration_alertmanager)	4m 12s
integration (integration_backward_compatibility)	10m 20s
integration (integration_memberlist)	8m 0s
integration (integration_querier)	15m 38s
integration (integration_ruler)	5m 40s
integration (integration_query_fuzz)	4m 52s
deploy_website	0s
deploy	0s
	1h 37m 20s

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