Ruler: Add support for per-user external labels #10424

Job	Run time
build	4m 1s
CodeQL	6m 45s
lint	4m 22s
test	15m 50s
integration-configs-db	2m 42s
integration (requires_docker)	7m 13s
integration (integration_alertmanager)	4m 8s
integration (integration_backward_compatibility)	10m 44s
integration (integration_memberlist)	7m 47s
integration (integration_querier)	15m 18s
integration (integration_ruler)	5m 32s
integration (integration_query_fuzz)	4m 28s
deploy_website	0s
deploy	0s
	1h 28m 50s

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