[ruler] add possibility for alert_relabeling config

[ToonTijtgat2]

Is your feature request related to a problem? Please describe.

We'd like to move our alerting from prometheus to mimir ruler and alertmanager to have all the alerts in one place. But there is no possibility to relabel before sending the alert to alertmanager.

Describe the solution you'd like

We would be in need to be able to do relabeling for alert. https://prometheus.io/docs/prometheus/latest/configuration/configuration/#alert_relabel_configs This is possible in the ruler of loki, but not for mimir. https://github.com/grafana/loki/blob/c350641a64b041228616c84c69e6fe834b5bd581/docs/sources/configure/_index.md?plain=1#L1019

[oferziss-armis]

+1

[fahdessmaki]

Please could you merge this PR ? Thank you 🙏