openrtb-core/src/main/java/com/google/openrtb/json/OpenRtbJsonFactory.java
public final <T> OpenRtbJsonFactory register(OpenRtbJsonExtWriter<T> extWriter,
      Class<T> extKlass, Class<? extends Message> msgKlass) {
    return register(extWriter, extKlass, msgKlass, FIELDNAME_ALL);
  }
public final <T> OpenRtbJsonFactory register(OpenRtbJsonExtWriter<T> extWriter,
      Class<T> extKlass, Class<? extends Message> msgKlass) {
    return register(extWriter, extKlass, msgKlass, FIELDNAME_ALL);
  }
Register an extension writer, not bound to any a field name (so this serializer
can be used for any extension of the provided class).

@param extWriter code to serialize some {@code extKlass}'s properties
@param extKlass class of extension message, e.g. {@code MyImp.class}
@param msgKlass class of container message, e.g. {@code Imp.class}
@param <T> Type of value for the extension
@see #register(OpenRtbJsonExtWriter, Class, Class, String)