This PR extends the repository integrity health indicator to cover also unknown and invalid repositories. Because these errors are local to a node, we extend the `LocalHealthMonitor` to monitor the repositories and report the changes in their health regarding the unknown or invalid status.
To simplify this extension in the future, we introduce the `HealthTracker` abstract class that can be used to create new local health checks.
Furthermore, we change the severity of the health status when the repository integrity indicator reports unhealthy from `RED` to `YELLOW` because even though this is a serious issue, there is no user impact yet.
This troubleshooting guide is what will be returned from the SLM health indicator
when a SLM policy has suffered from too many repeat failures without a successful
execution.