Ruler: Add support for per-user external labels #10403

Job	Run time
CodeQL	6m 46s
CodeQL	6m 46s
test	36m 20s
test	15m 8s
lint	4m 26s
lint	4m 26s
build	4m 1s
build	4m 1s
integration (requires_docker)	7m 14s
integration (requires_docker)	7m 14s
integration-configs-db	2m 33s
integration-configs-db	2m 44s
integration (integration_alertmanager)	4m 15s
integration (integration_alertmanager)	4m 15s
integration (integration_backward_compatibility)	10m 24s
integration (integration_backward_compatibility)	10m 24s
integration (integration_memberlist)	7m 41s
integration (integration_memberlist)	7m 41s
integration (integration_querier)	15m 22s
integration (integration_querier)	15m 22s
integration (integration_ruler)	5m 52s
integration (integration_ruler)	5m 52s
integration (integration_query_fuzz)	4m 17s
integration (integration_query_fuzz)	4m 36s
deploy_website	0s
deploy_website	0s
deploy	0s
deploy	0s
	3h 17m 40s

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