Alerting: New API endpoint GET /api/v1/ngalert (#55134)

* declare new endpoint /api/v1/ngalert
* add authorization for new path
* add request handler for the new path

pkg/services/ngalert/api/authorization_test.go

@@ -49,7 +49,7 @@ func TestAuthorize(t *testing.T) {
 		}
 		paths[p] = methods
 	}
-	require.Len(t, paths, 39)
+	require.Len(t, paths, 40)
 
 	ac := acmock.New()
 	api := &API{AccessControl: ac}