Add grease tokens

[Auth Scheme specification](https://www.ietf.org/archive/id/draft-ietf-privacypass-auth-scheme-15.html#section-4.1) presents a Greasing section in 4.1.

The library should provide a Greasing token type, along with associated test vectors. This would allow for a healthier ecosystem, and an easier implementation of greasing by origins.