Bug fix: Keep Constructor for GroupReturnCode class when using Proguard

Author: philips77

mcumgr-core/src/main/java/io/runtime/mcumgr/response/HasReturnCode.java

@@ -1,5 +1,6 @@
 package io.runtime.mcumgr.response;
 
+import com.fasterxml.jackson.annotation.JsonCreator;
 import com.fasterxml.jackson.annotation.JsonProperty;
 
 import org.jetbrains.annotations.NotNull;
@@ -18,6 +19,9 @@ class GroupReturnCode {
         @JsonProperty("rc")
         public int rc;
 
+        @JsonCreator
+        public GroupReturnCode() {}
+
         @Override
         @NotNull
         public String toString() {