Expose additional errors on invite batch (#141)

So we can see which users failed on the invite sent and which not

src/main/java/com/descope/model/user/response/UsersBatchResponse.java

@@ -1,6 +1,7 @@
 package com.descope.model.user.response;
 
 import java.util.List;
+import java.util.Map;
 import lombok.AllArgsConstructor;
 import lombok.Data;
 import lombok.NoArgsConstructor;
@@ -11,4 +12,5 @@
 public class UsersBatchResponse {
   private List<UserResponse> createdUsers;
   private List<UserFailedResponse> failedUsers;
+  private Map<String, String> additionalErrors;
 }