Support datasource-managed alert rules and contact points #354
Description
At the moment, the alerting tool only has support for Grafana-managed alert rules. In a setup where we use an external Alertmanager (i.e. Prometheus Alertmanager), we should support these alert rules as well.
As far as implementation, Grafana seems to call the /api/prometheus/{datasource}/api/v1/rules and /api/alertmanager/{alertmanager}/config/api/v1/alerts for this data to show up in the UI. Ideally these should be added to the existing alert tools but may make more sense as separate tools fetching rule groups under the respective datasources, or as a datasource proxied mcp server.