support opaque refresh tokens

[tobiasdcl]

Would you be open to the idea of supporting opaque refresh tokens?

If so I can submit a PR simular to #189 to add the option useOpaqueRefreshTokens 🚀

[jasonraimondi]

I think this is a reasonable add, I'd have no problem adding support for opaque refresh tokens. Happy to review and merge a PR for this feature!

[jasonraimondi]

I'll wait to cut a release for this PR

[tobiasdcl]

PR is ready #196 🚀