monitoring: Add Grafana Loki HR and Flux logs dashboard

- add loki-stack HelmRelease to install Loki and Promtail in the monitoring namespace
- make the loki-stack HelmRelease depend on kube-prometheus-stack to install Loki's datasource and service monitors in the correct order
- add a Grafana dashboard for displaying and filtering the Flux controllers JSON logs

Signed-off-by: Stefan Prodan <stefan.prodan@gmail.com>