Feature request: add support for the /delegation API

I'd like to use this SDK to obtain an access_token for a refresh_token - it looks like in Auth0 this is done with the [/delegation API](https://auth0.com/docs/api/authentication#delegation). Is it possible to add that to the Java SDK so I don't have to roll my own implementation? Thanks!