Switch to native generator

Currently the default generator cannot properly process parameter explode. During testing the cleverdata client and server, we found the client failed to send out the correct form of request. And the investigation shows that the generator needs to be changed.

This MR switched the generator to the native generator. It requires at least Java 11, so all clients using the generated lib will need to be at least Java 11, but it fixes the issue and offers some benefits like fewer dependencies to maintain.

The native generator has been deployed in the cleverdata api for a while, and we found no issues with this generator. I think it's the best solution with the minimal effort required to address the problem.