Render ThanosRuler instances

[mdnix]

Summary

Adds thanosRuler as an instance component, so an instance can render a ThanosRuler next to its Prometheus and Alertmanager. Enabling it emits the ThanosRuler, a ServiceAccount, a ServiceMonitor and a PrometheusRule with a few guard alerts.

I went back and forth on where this belongs. prometheus-operator installs the CRD and this component already passes --thanos-ruler-instance-namespaces for every instance namespace, it just never creates a ruler no logic for that in kube-prometheus. There is also kube-thanos which has a rule module but emits a plain StatefulSet with ConfigMap rule files and no PrometheusRule discovery and its last release was about four years ago. I also looked at a separate component around the Helm chart but that chart always wants either object storage or remote-write, which I do not need in my case, I only want the ruler to evaluate alerting rules and hand them to Alertmanager.

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[github-actions]

🚀 This PR has been released as v2.19.0

Triggering workflows Release

🛠️ Auto tagging enabled with label bump:minor

[simu]

LGTM now.