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.
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