Add requestHeaders field for Appsync lambda authorizer event

Author: Xiang Gong

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

@@ -30,6 +30,7 @@ public class AppSyncLambdaAuthorizerEvent {
 
     private RequestContext requestContext;
     private String authorizationToken;
+    private Map<String, String> requestHeaders;
 
     @Data
     @Builder(setterPrefix = "with")