Add Pebble readiness and liveness checks to all COS charms with their status reflected in Juju status

[gaborbk-canonical]

When the COS charm workloads encounter issues or break for any reason, the Juju status of the charms still shows the workloads as healthy. This is misleading, and can be fixed by adding Pebble readiness and liveness checks to all COS charm workloads, reflecting their status in Juju status.

For example, Grafana shows healthy in Juju status in my cluster while the workload is erroring:

kubectl -n my-cos-model exec -ti grafana-0 -c grafana -- pebble services
Service  Startup  Current  Since
grafana  enabled  backoff  today at 10:58 UTC
kubectl -n my-cos-model exec -ti grafana-0 -c grafana -- pebble checks
Plan has no health checks.