Correct the type for challengeAnswer attribute. (#356)

Author: msailes

aws-lambda-java-events/src/main/java/com/amazonaws/services/lambda/runtime/events/cognito/CognitoUserPoolVerifyAuthChallengeResponseEvent.java

@@ -72,7 +72,7 @@ public static class Request extends CognitoUserPoolEvent.Request {
         /**
          * The answer from the user's response to the challenge.
          */
-        private Map<String, String> challengeAnswer;
+        private String challengeAnswer;
         /**
          * This boolean is populated when PreventUserExistenceErrors is set to ENABLED for your User Pool client
          */
@@ -81,7 +81,7 @@ public static class Request extends CognitoUserPoolEvent.Request {
         @Builder(setterPrefix = "with")
         public Request(Map<String, String> userAttributes,
                        Map<String, String> clientMetadata,
-                       Map<String, String> challengeAnswer,
+                       String challengeAnswer,
                        Map<String, String> privateChallengeParameters,
                        boolean userNotFound) {
             super(userAttributes);

aws-lambda-java-events/src/test/resources/event_models/cognito_user_pool_verify_auth_challenge_response_event.json

@@ -19,9 +19,7 @@
     "privateChallengeParameters": {
       "secret": "11122233"
     },
-    "challengeAnswer": {
-      "string": "123xxxx"
-    },
+    "challengeAnswer": "123xxxx",
     "clientMetadata": {
       "exampleMetadataKey": "example metadata value"
     },