Allow drop-in states language

There are gaps in the SDK and as more services and features are adding, updates to states language is going to continue to happen before this repo. There should be a way to easily patch in your own additions (such as ResultSelector) in the interim period.

The limitations on this SDK could be justified as "validation" which is a great thing to have prior to deployment. However, field constraints should only inhibit those who want to validate classes themselves. Extending the library is too much to ask for when folks just need things to work and know that it's supported in states language.