add a new task to generate code and test the generated app

[stevehu]

Recently, we have made some enhancements to the light-codegen to resolve some issues and add new features. As a lot of our customers are using the tool to migrate to new light-4j releases, they found some regenerated models are not the same and existing code needs to to be updated to work with the new model classes.

The current light-codegen only has unit tests and it won't cover all use cases from our customers, in order to ensure that any update won't break customers code generation, we are going to add a new task in light-bot to call the light-codegen. Once the app is generated, we can test it against the example response defined in the specs.

Once this is done, we will encourage our users to submit their specs here and update the tests to ensure that their specifications won't be broken with new changes. For users who have internal API and don't want to reveal the specs, it is recommended to submit a stripped down version instead.