ruler: Support alert relabeling

[badrabubker]

Hello Loki development team,
I would like to ask for adding alert_relabel_configs to ruler

Is your feature request related to a problem?
I have the current setup:
Prometheus + Alertmanager + Opsgenie and recently Loki was deployed
I use alert_relabel_configs in Prometheus to map severity of an alert to Opsgenie priority
e.g.

alert_relabel_configs:
  - source_labels: [severity]
    regex: emergency
    target_label: opsgenie_priority
    replacement: P1

to abstract Opsgenie priorities. I can't do that with ruler since it does not support alert relabeling.

Describe the solution you'd like
Add alert relabeling similar to Prometheus

Describe alternatives you've considered
I could do the mapping on alert manager side but I had to define 5 more receivers for each receiver I had
e.g.

      routes:
      - match:
          severity: emergency
        receiver: events-P1
      - match:
          severity: alert
        receiver: events-P2
.....

Additional context