Configurable alerts in Grafana dashboard

[DifferentialOrange]

Now we have an example of alerts for Prometheus that one may use. But using Grafana alerts is more widespread among Tarantool Grafana dashboard users. For now, there wasn't any support for Grafana alerts since the main product of this repo -- published dashboards for InfluxDB, Prometheus, InfluxDB TDG, Prometheus TDG -- use dynamic variables, which cannot be used with alerts. Since we added static build support, it is possible to support them for static built dashboards.

@palage4a already had a prototype to solve Delivery team needs. The idea is as follows: since thresholds may be different for each application, all alerts are specified in dashboard configuration. We'll need to find a balance between predefined set of alerts and customization and implement them here, in common purpose library.