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/api/authorization_test.go

@@ -40,7 +40,7 @@ func TestAuthorize(t *testing.T) {
 		}
 		paths[p] = methods
 	}
-	require.Len(t, paths, 60)
+	require.Len(t, paths, 62)
 
 	ac := acmock.New()
 	api := &API{AccessControl: ac}