Add a new library template to Java client: okhttp-gson

which uses OkHttp as HTTP library and Gson as JSON processing library

modules/swagger-codegen/src/main/resources/Java/StringUtil.mustache

@@ -39,4 +39,13 @@ public class StringUtil {
     }
     return out.toString();
   }
+
+  /**
+   * Convert the given object to string with each line indented by 4 spaces
+   * (except the first line).
+   */
+  public static String toIndentedString(Object o) {
+    if (o == null) return "null";
+    return o.toString().replace("\n", "\n    ");
+  }
 }