add using Azure.Core to mvad (#23384)

Author: iscai-msft

specification/cognitiveservices/AnomalyDetector/multivariate/routes.tsp

@@ -5,6 +5,7 @@ import "./models.tsp";
 
 using TypeSpec.Rest;
 using TypeSpec.Http;
+using Azure.Core;
 
 namespace AnomalyDetector.Multivariate;