Add in new `states` Interface service.

[willfarrell]

https://aws.amazon.com/about-aws/whats-new/2019/12/aws-step-functions-now-supports-aws-privatelink/

[github-actions]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues. If you have found a problem that seems similar to this, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.