Alerting: Provisioning API respects global rule quota (#52180)

* Inject interface for quota service and create mock

* Check quota and return 403 if limit exceeded

* Implement tests for quota being exceeded

pkg/services/ngalert/models/alert_rule.go

@@ -23,6 +23,7 @@ var (
 	ErrRuleGroupNamespaceNotFound         = errors.New("rule group not found under this namespace")
 	ErrAlertRuleFailedValidation          = errors.New("invalid alert rule")
 	ErrAlertRuleUniqueConstraintViolation = errors.New("a conflicting alert rule is found: rule title under the same organisation and folder should be unique")
+	ErrQuotaReached                       = errors.New("quota has been exceeded")
 )
 
 // swagger:enum NoDataState