Alerting: Implement /status for the notification system (#33227)

* Alerting: Implement /status for the notification system

Implements the necessary plumbing to have a /status endpoint on the
notification system.

* Add API examples

* Update API specs

* Update prometheus/common dependency

Co-authored-by: Sofia Papagiannaki <sofia@grafana.com>

pkg/services/ngalert/api/api.go

@@ -25,6 +25,7 @@ var timeNow = time.Now
 type Alertmanager interface {
 	// Configuration
 	SaveAndApplyConfig(config *apimodels.PostableUserConfig) error
+	GetStatus() apimodels.GettableStatus
 
 	// Silences
 	CreateSilence(ps *apimodels.PostableSilence) (string, error)