update request transformer to do some validation logic

[stevehu]

While migrating the SAG gateway to the light gateway for one of our customers, we encounter some extra validations that are not standard. In order to avoid any consumer application change, we need to mimic these behaviours. The best place we can do that with maximum flexibility is the request transformer. We can ask the project team to create a transformer rule action class and plugin to the request/response chain to do the special validation. If any validation rule is failed, it will stop the chain and return an error message with the designated HTTP code to the consumer.