Alerting: Extract alerting rules authorization logic to a service (#77006)

* extract alerting authorization logic to separate package
* convert authorization logic to service

pkg/services/ngalert/accesscontrol/models.go

@@ -0,0 +1,9 @@
+package accesscontrol
+
+import (
+	"errors"
+)
+
+var (
+	ErrAuthorization = errors.New("user is not authorized")
+)