Provide support for JSR-303 annotations

[kkrauth]

Enable ability to turn on JSR-303 bean validation annotations for the generated model.

[wing328]

@kkrauth thanks for the feedback. Would you have cycle to contribute the enhancement?

Ref: https://github.com/swagger-api/swagger-codegen/blob/master/CONTRIBUTING.md

[wing328]

Discussion moved to #2549