Alerting: Time intervals API (read only endpoints) (#81672)

* declare new API and models GettableTimeIntervals, PostableTimeIntervals
* add new actions alert.notifications.time-intervals:read and alert.notifications.time-intervals:write.
* update existing alerting roles with the read action. Add to all alerting roles.
* add integration tests

pkg/services/ngalert/accesscontrol.go

@@ -25,6 +25,9 @@ var (
 					Action: accesscontrol.ActionAlertingRuleExternalRead,
 					Scope:  datasources.ScopeAll,
 				},
+				{
+					Action: accesscontrol.ActionAlertingNotificationsTimeIntervalsRead, // This is needed for simplified notification policies
+				},
 			},
 		},
 	}
@@ -109,6 +112,9 @@ var (
 					Action: accesscontrol.ActionAlertingNotificationsExternalRead,
 					Scope:  datasources.ScopeAll,
 				},
+				{
+					Action: accesscontrol.ActionAlertingNotificationsTimeIntervalsRead,
+				},
 			},
 		},
 	}