Ruler: Add support for per-user external labels #10396

Job	Run time
test	15m 47s
lint	2m 33s
CodeQL	6m 40s
build	3m 59s
integration-configs-db	2m 28s
integration (requires_docker)	7m 15s
integration (integration_alertmanager)	4m 14s
integration (integration_backward_compatibility)	10m 31s
integration (integration_memberlist)	8m 6s
integration (integration_querier)	15m 27s
integration (integration_ruler)	5m 51s
integration (integration_query_fuzz)	4m 46s
deploy_website	0s
deploy	0s
	1h 27m 37s

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