fix: WebhookCustomHeader does not extend EasyPostResource

Author: Justintime50

src/main/java/com/easypost/model/WebhookCustomHeader.java

@@ -3,7 +3,7 @@
 import lombok.Getter;
 
 @Getter
-public final class WebhookCustomHeader extends EasyPostResource {
+public final class WebhookCustomHeader {
     private String name;
     private String value;
 }