Alerting: Rule Modify Export APIs (#75322)

* extend RuleStore interface to get namespace by UID
* add new export API endpoints
* implement request handlers
* update authorization and wire handlers to paths
* add folder error matchers to errorToResponse
* add tests for export methods

pkg/services/ngalert/api/authorization_test.go

@@ -49,7 +49,7 @@ func TestAuthorize(t *testing.T) {
 		}
 		paths[p] = methods
 	}
-	require.Len(t, paths, 50)
+	require.Len(t, paths, 52)
 
 	ac := acmock.New()
 	api := &API{AccessControl: ac}