Alerting: Fix rule API to accept 0 duration of field For (#50992)

* make 'for' pointer to distinguish between missing field and 0
* set 'for' to -1 if the value is missing but not allow negative in the request + path -1 with the value from original rule
* update store validation to not allow negative 'for'
* update usages to use pointer

pkg/tests/api/alerting/api_admin_configuration_test.go

@@ -161,7 +161,7 @@ func TestAdminConfiguration_SendingToExternalAlertmanagers(t *testing.T) {
 			Rules: []apimodels.PostableExtendedRuleNode{
 				{
 					ApiRuleNode: &apimodels.ApiRuleNode{
-						For:         interval,
+						For:         &interval,
 						Labels:      map[string]string{"label1": "val1"},
 						Annotations: map[string]string{"annotation1": "val1"},
 					},