[mimir-distributed] Add additionalRuleLabels to PrometheusRule alerts

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

Custom labels added to alerts are always useful and are often used for routing purposes. The mimir-distributed helm chart allows the user to deploy default PrometheusRules (alerts and recording rules). However, no option exists to add a custom label to all the alerts (and rules?) deployed.

Describe the solution you'd like

Much like it is the case for other helm charts like Loki, a posibility would be to create and configure a key like:

metaMonitoring:
  prometheusRule:
    additionalRuleLabels:
      custom_label: custom_value

Describe alternatives you've considered

Unfortunately with my knowledge I currently don't see other alternatives.

Additional context

Even if the structure is different, an example of this feature exists for the Loki helm chart: https://github.com/grafana/loki/pull/9020